Hi Helen! The size of ball is actually determined by your height rather than the type of exercises used. Being that you say you are in need of a NEW one, does that mean you already owned one? If so and you liked it, I would order the same one. If you are not sure what size that was then it is best to go woth the general guidelines for a starting point. I find 55cm and 65cm balls to be the most popular size choices. Usually 55cm is good for a height of up to 5'6" or 5'7" and then 65cm for up to 5'11".
Helpful Hint: If after you have used your ball, you are finding the exercises way too difficult due to balance issues, let a little air out of your ball (creating more of a stable base) until you get better at the exercises.
Good Luck!