How do pro Starcraft 2 tournament organisers choose maps?

So in a small scale tournament, one of the large considerations is what maps people can practice on.

While anyone can practice on any map (unless they have Starter Edition), most people lack a large stable of practice partners. As a result they are often restricted on what maps they'll be able to get decent amounts of practice. If you look at the After Hours Gaming League you'll notice that Day9 specifically chose Blizzard ladder maps for his tournament. This was because many of the players (despite being Masters level) had difficulty organizing their teams for long practice sessions. As a result many of the practice sessions would have to be alone and there for on the Blizzard ladder.

While an argument was originally made for GSL/MLG/TSL maps because you could organize a random custom game on them (part of Battle Net match making), the quality of the players you were matched against could not be guaranteed. Un-like the iCCup of the Broodwar era, where the player base was more organized and you could guarantee the skill level of your opponents, the popularity of the Blizzard ladder leaves it the only real option for people who don't have a good organized practice team.

I think this is an important concern for any tournament organizer looking to start up a friendly tournament.

There is a balance between maps that are balanced and fun to play/watch. The best way to produce a balanced map would be a perfect symmetrical map with few elements in there, but that wouldn't be fun to play (and even if you go the symmetrical route, there are still things to consider. For example, tight chokes with a ramp tend to favour terrans).

Then you have to consider how overplayed is a map. After some time, builds then to stabilize for most maps and it can lead to repetitive games.

However, for a friendly tournament I would not give too much thought to this. I'll go with the map pool used for the 1v1 ladder, which has the advantage that is probably well known to more casual players. Then it depends on how you want to structure it (like say, fixed maps for group play, and then for the elimination round the first map is fixed and then the loser chooses). You should probably consult with some of your players if you can.

I would suggest looking at the TLPD here: http://www.teamliquid.net/tlpd/sc2-international/maps and choosing maps which are relatively well balanced across all matchups and have been played recently (within the last week or two).

Many tournaments have essentially their own pool of mapmakers as a team to design future maps. iCCup and GSL are the two major ones, and then of course Blizzard. MLG tends to just take other versions and make small tweaks, they haven't made any themselves that I'm aware of.

I personally would choose a varied pool of about 7-8 maps, picking mostly those that are shared across leagues or in the ladder, so your players are likely familiar with them. If you know most of the people involved, you should also know what leagues they watch, if any, and that should influence your decision. Allow each player to thumb a single map down for each set, and go from there.

Also, I'll point you to http://challonge.com/ as an excellent way to organize your tournament.