Recommended power bank capacity for iPhone 4S
The power bank capacity is just that, capacity. So the bigger the capacity, the more often you can recharge your iPhone before you need to recharge your power bank. Give or take a few mAh getting lost on the way, the number of recharges possible is basically Capacity of power bank divided by Capacity of iPhone.

This also means that there is no recommended capacity per se. Questions you may want to ask yourself before buying one include


do I need to recharge several time a day or is one recharge usually enough?
do I plan to take multi-day cross-country hikes without power outlets on the way
is low weight important for me (more capacity usually means more weight)
do I just want to recharge one device or is the goal to charge e.g. an iPad as well