I have a small one and medium one. The small holds most of my work stuff, including books, lunch, etc. It would fit a 6-pack of beer, maybe 2. On most days, the small is large enough for me. I feel like the medium is HUGE. Holds all of the above, including a change of clothes and then some. 2, maybe 3 6-packs.

I would say, if you're commuting and carrying a lot of stuff, go for the medium. IMHO, there's no need for a large unless you're a messenger. You might want to go to REI or Performance and check out what they have to give you a better idea of what you want.

Hope that helps!