I should have said half-circumference not half diameter ;-)
Anyway, the ball I have had a chart on the box that said:
Ball Diameter Height
55cm 4'8" to 5'3"
65cm 5'4" to 6'0"
75cm 6'1" to 6'7"
but I'd just check whichever ball you end up looking at and seeing what they recommend. I saw something online for a different ball that said, "To see if a 55cm ball is right for you, place a mark on a wall about 20 inches high (16 in. for a 45cm ball), and squat next to it. Consider that you will sink down a few inches (depending on your weight and amount of inflation). This is approximately how tall the ball will be when sitting on it. Get a larger size if you don't like this height"
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