8G RAM and SSD - how big should the swap be?

You should be fine with just 2 or 4 Gb of swap size, or none at all (since you don't plan hibernating).

An often-quoted rule of thumb says that the swap partition should be twice the size of the RAM. This rule made sense on older systems to cope with the limited amount of RAM; nowadays your system, unless on heavy load, won't swap at all.

It mostly depends whether you're going to do a memory-intensive use of your machine; if this is the case, you might want to increase the amount of RAM instead.

Note that a SSD is subject to more wear and tear than a hard disk, and is limited by a number of rewrite cycles. This makes it not optimal to host a swap partition.

Edit: Also see this question: Linux, SSD and swap

Assuming we are not rehashing Is swap an anachronism? conversations/debates, I would suggest you take a look at the archlinux wiki on the topic. As always it is a great resource for any distro and covers the basics as well as suggested performance tuning for ssd's.

The tldr; according the the arch wiki:

Swap space is generally recommended for users with less than 1 GB of RAM, but becomes more a matter of personal preference on systems with gratuitous amounts of physical RAM (though it is required for suspend-to-disk support).