Before revision: An average user should be able to start with a 45 nbspmm (1.8 in) ball and move up to 60 nbspmm (2.4 in) as their muscles get accustomed to the exercise.
After revision: An average user should be able to start with a 45 mm (1.8 in) ball and move up to 60 mm (2.4 in) as their muscles get accustomed to the exercise.