I was looking for a general guideline as I am to purchase an exercise-ball online. According to this article:
Knees should be level or slightly lower than the pelvis - creating an angle of 90 degrees or slightly greater at the hips and knees (thighs parallel to ground or pointing down slightly).
They also give a general guideline on choosing the ball for your height, if you are heavier than average you have to take that into account, too: (cm added by me).
45 cm 5' and under | up to 152.5cm
55 cm 5'1"– 5'8" | up to 172.5cm
65 cm 5'9"– 6'2" | up to 188cm
75 cm 6'3"– 6'7" | up to 200.5cm
85 cm 6'8" and taller | above 200.5cm
You shouldn't buy a ball that is too big for you:
Bear in mind, releasing air from the exercise ball will cause it to lose air pressure. As the ball flattens out, this will actually make it more stable, as it has a larger contact area with the resisting surface and the body. This means that stabilizing and balancing exercises will become easier and will lose some effectiveness.