10X10 seems to be the norm for size,
After that, it's all on how much you want to spend. Straight leg or slant leg? Most everyone I know that uses an E-Z-Up (or simular style), uses the straight leg ones. Much easier to put up a bunch close together for more pit area if you have guys you usually pit with.
I've been usuing the First-Up ones from Wal Mart. Not too expensive and have some walls you can put up that are fairly cheap. The big draw back is that they aren't built super tuff and if in a high wind area, can get beat up pretty quick. More so if you have all the walls on. I've had it happen a couple of times.