The height requirements for the stability balls are just general guidelines. Leg length is really what matters when choosing the correct size ball. If you have long legs, then you might need a larger ball. I'm 5'7" but need a 65cm ball because I have long legs. You want your hips to be level with or slightly higher than your knees.
In addition, as some other people pointed out, not all brands of stability balls blow up the same. I purchased the 65cm burst resistant version of the GoFit and TKO stability balls on Amazon.com (on sale + free shipping w/$25) and I really like them.
Hope that helps.