kid, go to the chemist and buy the smallest boxes of as many brands as they have. it's like buying shoes. you've gotta try them on to get the perfect fit. think of how much more enjoyable it is to walk in well fitting, comfortable shoes!
...plus, you need to practise putting them on right. fumblings & failed attempts to roll one on can be an erection killer, and a condom that's not put on right won't give full protection.