Stability Ball-- Which one???

So I just recently got introduced to the stability ball and tried it out at my gym. Its fantastic resistance training. I'm confused though. I'm female, 5 3", 160 pounds and the 55cm ball is the one that is recommended. BUT when I sit on it my knees are higher than my hips.. which from what I hear is all wrong. The 65cm ball is perfect but its recommended for people 5 4" and above.. Any advice on which one I should continue with ? I don't want to hurt myself.(I already have back problems.)

Realize the recommendations are based on averages.  If your legs are longer than "average", then you'll need the bigger ball.

Go with whats comfortable.

Oh thank god, because I hate the one recommended for my height, too.  I go one higher because it hurts to use the smaller one (bad "you're doing it wrong" hurt). 

Thanks... thats good to hear.. Yea I may have longish legs..(chubby ones at 160 ;)) Guess I'll stick with whats comfortable.

It may also be that the 55 cm ball is not as inflated as it could be.  Stability balls should be pretty firm and not soft and squishy.

thank you for this post-I purchased the "recommended" size for me and hate it.  I sit down and my knees are above my hips and found it to be very uncomfortable to use.