Stability ball question

This is a question for regular users of stability ball workouts.

I am 5'6" and work out 5-6 days a week, in decent shape, but new to stability ball work. I purchased a 65cm ball on the advice of a chart I found online. Today I made my first attempt to do a stability ball workout, and it was, to say the least, unsatisfactory. I kept comparing the way the girls on the workout looked in relation to their ball with the way I was fitting onto mine. I kept rolling off the thing, and I couldn't seem to reach the floor in the same way that the leader was doing. I think my ball is too large for my height, but it could just be that the newness of it all is the problem. I hate that I spent a pretty good amount of money on this blooming thing only to have to buy a new one, but on the other hand, I don't want to hurt myself using the wrong equipment.

Any thoughts or advice?

Hmm.  Well we have our own and I think it is probably too big for me but really my DH got it as a present so I don't use it much.  He uses it quite a lot as well as using the one at the gym.  At our gym I use the small or medium sized one when I do stability ball exercises (which is not that often I admit).  But I am a lot shorter than you. 

I think one good test is sitting on it like a chair.  If your thighs are not parallel to the ground, and you have to slant your legs downward to put your feet on the ground, then it is probably too big? 

DH actually tried to JUMP on the one at the gym last week in an attempt to stand on it!  Ha ha ha ha I was running on the treadmill and happen to look over at him and I actually see him do this and immediately roll off and fall over.   I knew he hadn't hurt himself so I just laughed.     He said he had done it before (stood on the ball) in circus training but I told him it was probably not the same type of ball.  He admitted that yes it wasn't.     Sometimes DH is just hilarious.

Wow you work out 5-6 times a week?  That is great - congrats.  I am still trying to get up to my 4 times a week running for the marathon and one cross training.  Stupid office work is conspiring against me.   

I am the same height as you Carla and just bought the 65cm ball. I ended up needing to fill it up to nearly 75cm because at 65cm it was too squishy. At around 70cm it is actually perfect. There must be 55cm balls, right? If you have a Gaiam store near you maybe check there.

It also takes quite a while to get used to the darn thing - I work with a trainer with one and still slide all around on it...it's the right height, I'm just not that good at using it...but I'm getting better!

Your knees should be in line with the top of the ball. I am only 5' 3'' and require a 65 cm ball. The 55 was too small. The balls vary as to how much they can stretch when they are pumped up. The brand I have is GAIAM. Related to this topic, I'm very curious to know if places in the UK offer classes (Fit Ball, Stability ball, aka Resistaball). I've been in contact with resistaball trainers and they say they have not branched out to the UK yet. This is my absolute favorite exercise class.