Theoretically there isn't a limit how much power you can get out of the battery pack. Whether it will be portable is another question.
I personally use a rebranded Lenmar 6.6Ah pack (1x USB 1A, 1x USB 2.1A) I found in a tigerdirect retail store's bargain section. Looks like a slightly expanded deck of playing cards. Works quite well, getting 3-4 full recharges for my iPhone between charges.