The 2oz is the minimum size I would carry, those little keychain ones arnt very useful.
If your just going to put it in your pocket I would recommend the flip-top to prevent accidental discharge. The "cop-top" is basically just an open-top with no safety system to keep you from pressing the button by accident (but its easier to use in a stressful situation). You can see the differance here:
http://www.foxlabs.net/Products.html
The only other thing you have to consider is to get a stream or fog pattern. Stream is best in windy condition and has better distance, fog is good because you dont have to aim as well to get a hit but the spray can blow back into your face under windy conditions.
I would suggest you get a holster anyways rather than pocket carry. You can find a good one for the foxlabs 2oz cans here:
http://www.comp-tac.com
And I would get the cop-top fog pattern here:
http://www.selfdefenseproducts.com/sprays.htm
(buy two cans, use one to practice with and become familiar with the spray pattern)