Exercise! - Stability Ball

I just bought a stability ball and it's the "medium" size. It was all they had but the box doesn't specify what size person fits the medium ball. It's 64cm. The one ball I had several years ago looked bigger but said medium.

Just curious what size I should be using...

CW~205
HT~5'6

TIA~

55 cm. ball

21 inches

4' 11" - 5' 4" tall

65 cm. ball

25 inches

5' 5" to 5' 11" tall
The standard method for determining proper ball size is by sitting on the ball. For most exercise applications, there should be a 90 degree angle at both the hip and knee joints and your thighs should be parallel with the floor when seated on the ball.

Hope this helped..
Heather<><

I've been thinking of getting one of these, but the kits-the ones with a ball and a video- I see don't mention what size ball comes with it. Is it safe to assume that they are medium size?

I need a small ball. Being short can be such a hassle sometimes.
Thanks
Sarah