Is there a way to increase inventory size in Diablo II?
No. Without third party mods, there is no way to increase the size of your inventory or your stash.
There are a few tricks you can do, however, to take creative advantage of some game mechanics. Try carrying the Horadric Cube with you, which has a 3x4 area inside it but only takes up a 2x2 area in your inventory. You can also equip items you're not using. This works best with weapons, since you can swap to the unused weapon set, equip the item, then swap back to the weapon you actually want to use.
Another trick is that you can open your inventory and pick up an item off the ground even if your inventory is full. If you quit the game while this item is replacing your cursor, the next game you join will still have that item available to you. You can use this in a multiplayer game to ninja an important item even if your inventory is full.
Even with these minor space savers, you still won't be able to pick up everything. You'll just have to learn to live with leaving some items on the ground. They're not all worth grabbing.
You can't increase the size of your inventory or chest. However, there are many tricks that new players don't know about:
Big belt. Anything that isn't a Sash or Light Belt will have 12 potion slots (3 per hotkey) and Plated Belts (and all Exceptional/Elite Belts) give you 16. Typically, the belt will auto-load as you pick up potions; if you have an empty hotkey slot, the potion goes in it, and if you have empty slots above the hotkey slots, potions of the same type (not necessarily the same size) will auto-load into them. Maintain your belt storage; start with a 2/2 mix of healing and mana, and if you feel you need more of one or the other, go 3/1 in the necessary direction. Put the biggest ones you have in the belt, biggest first (but don't waste big potions on small boo-boos; if you're on the cusp HP or MP-wise between two levels of potion, one isn't enough but the next is too much, then keep a column of both). Consider a Rejuvenation Potion column in higher difficulty levels; in addition to restoring both HP and MP, they do it instantly instead of gradually increasing it. Chances are there will be times when you're just plumb drained in both globes, especially if you're a "meat shield caster" like a Javazon, Hammerdin, Zealadin, or similar, where you're taking damage and spamming skills nonstop.
Tome of Town Portal. For two inventory slots, you can hold up to twenty Scrolls of Town Portal. This should be your first purchase as soon as you can afford it. Can't believe nobody has mentioned this one given your mention of scrolls. Don't bother with a Tome of Identify (keep reading for why).
Horadric Cube. 2x2 outside space, 3x4 in (gives you three times the storage space as what it takes up). Just be careful with the Transmute button; if you have just the right (or wrong) combination of items and accidentally hit that button, you lose what you had for something that may not be what you wanted. Potions, gems and runes in particular can be easily lost this way.
Minion. Your hireling can be equipped with a weapon, shield (if the weapon's one-handed of course), armor, helmet and boots, IIRC. Typically he should get your hand-me-downs, or any weapon better than what he carries but that you can't/won't wield. Keep in mind, what he's equipped with will never lose durability, so ethereal gear is still good hireling equipment if the bonuses are good.
Alternate Weapon Set. You can equip two weapon sets and switch between them. If you have a totally kickass primary weapon set that your character's built around, your backup is just extra inventory; put the weapon or weapon-shield combo that takes up the most space in your backup set (Tower/Gothic Shields, Long Swords, Dimensional Blades, Cutlasses, most bows, most two-handed melee weapons, and Heraldic/Aerin Shields if you happen to be a Paladin). Obviously you have to have sufficient strength or dexterity to wield these items, which for casters can be problematic.
Maintain your Chest. Your chest should carry only things you can trade with other players for big bucks, or that you have found or traded for but can't quite use to their full potential yet. Socketables (gems/runes/jewels) and good alternate equipment (like a set with more magic find but less of some other desirable quality than your primary equipment) are the order of the day for chest storage; keeping a backup supply of potions or scrolls here is a waste of space.
Town Portal often. If you're grinding your way through areas looking to build wealth/experience, it's common practice to mow down a mob, gather everything you can carry, TP, sell it all, restock any potions or scrolls you're low on and not finding (never fill your Tome from a merchant; keep about 5 TP scrolls handy at all times, but leave plenty of room to pick up any you find), and go back for more. This is especially true when it's someone else's Town Portal; they only disappear when the guy who cast it travels through it from the town to the battlefield. Until that happens, anyone else can use it as many times as they like. Usually, people playing in groups will figure out among themselves who'll use a TP scroll, then everyone uses one portal. Sometimes two players will each cast one, and then you have a semi-permanent way back as long as the two casters don't return through their own portals.
Prioritize. Spoiler alert (I won't put it in tags since the game's pretty darn old and you know the whole story before you're a third of the way through the full game): after rescuing Deckard Cain in Act 1, he will identify any items for you, for free, for the rest of the difficulty level (and if you really need to identify an item in Nightmare or Hell Act 1, just leave your game and create another in a lower difficulty level). This makes Scrolls of Identify nearly worthless except early in the game to sell. I have been known to buy a Tome of Identify to store scrolls; they're still useful for quickly identifying equipment like Uniques or Charms that could be useful immediately, and they basically double their value from then they're purchased to when you sell them after hitting 20 scrolls. However, it's not necessary.
Be careful with the size of the Charms you carry around; there are precious few Large Charms that give you a set of buffs you can't get just as easily with a Small Charm. Look to upgrade these, just as you would any other piece of equipment. The smaller the charm, the better, and make sure you're not carrying more than you need (your equipment's resistances only stack up to 75% unless you have an item that increases your max resistance, and do you really need that +2 to Attack Rating charm halfway through Nightmare?)
Beyond a certain point in your questing, certain items just don't make sense to pick up. By the time you're in Nightmare, you and your hireling should have good enough equipment and be making enough gold from high-quality drops, that if it's equipment and its label is white, it's usually garbage. Similarly, pretty much all of the offensive potions are garbage (your own attacks leave them all in the dust and they won't even really help against immunities), and the smaller healing/mana potions can be left behind as well, unless you're running really low and desperate. Even though your wallet is bottomless, I've even passed up Gold from time to time if getting to it would attract unwanted attention. Gold's useless for trading, and by Nightmare difficulty, you'll be finding (or trading for) virtually everything worth wearing anyway. Players will give you Gold if you're a little short trying to repair equipment. The only real use for large amounts of gold beyond Normal difficulty is gambling, and you will not break even long-term so most players consider it a waste of time and money versus straight dungeon-grinding. As long as you have enough money to repair all your equipment and restock your potions, you're doing alright.
Yes, there are mods that can do this for you. They're not against the TOS or EULA as long as you're playing offline, single-player mode.
PlugY is probably the best and most straightforward of these. It changes the stash so that:
- The stash becomes 10x10.
- The stash has over four billion pages (in other words, it's basically unlimited).
- There's a personal stash (per character) and a shared stash (shared among all the characters). Each has four billion pages.
It has various other features as well:
- You can unassign and reassign skill points at any time.
- You can unassign and reassign stat points at any time.
- It can automatically run the
players /Xcommand for you for each game. - It shows many extra stats such as % magic find.
- It displays item level for every item.
- It lets you open the cow level any number of times.
And even more. Again, if you're playing online, you're out of luck, but if you're playing single player, there's nothing wrong with using mods. The game is designed to let you use them.
Actually installing PlugY is a little bit confusing, but here's a video that shows you how to do it, step by step:
How to - Install PlugY for Diablo 2
Important note: With a four-billion-page stash, instead of the problem of what to keep and what to sell, you'll face a different problem: Where in these four billion pages is the item you're looking for!? You'll want to come up with some sort of scheme for how to organize your items before you get too far in.