Jazz, usually the "right" size is the one, properly inflated, that you can sit on with your thighs parallel to the floor, while your shins are at a right angle. So if you're a leggy 4'11", the 55 cm would probably be best. But if you have a longer torso, then the 45 cm might be better for you.
We use stability balls in pilates classes at my gym, & I've discovered that an undersized ball is better for core work when I'm using the ball above me and for Cathe's PLB floor work. I prefer the "right" size ball when I'm sitting on it or working above it, for movements like the "v pikes".
I truly enjoy using stability balls & BOSUs. It's hard to cheat, & I can tell by the sweat factor & heart rate, that I'm working a lot harder. The both allow for a much greater range of motion for some exercices. My hip flexors are very happy this week. The stability ball also allows for some truly decadent spinal stretching. It's a great addition to your fitness tools.
Debra