For use with The Core & The Floor most women who are 5’7 (170cm) or less will find a 55 cm ball is suitable.
Only women 5’8 (173cm) and above should need a larger, 60-65cm ball.
The best way to check the size is by actually sitting on the ball. When sitting on the centre of your ball your thighs should be parallel to the floor or with your hips slightly higher that your knees.