Debra I think the ball is 65 cm. A bit smaller or larger wouldn't affect the workout too much though so you can use what you have. If you are looking to order, I would go 65 cm.
Rajashree Srirangarajan most definitely!! I am a big fan of figuring out how to make a movement work for your body. The feet on the wall is a great choice as it helps to secure the position, creates stability and eliminates the feet slipping on the floor. Great question!!