This is an addendum to my previous comment. If you multiply the diameter (in this case 24 inches), by pi, 3.14, you will come up with the proper circumference for the ball (75 to 76 inches).
I drew up a quick and silly sketch that illustrates these steps:
- Measure a string or ribbon cut to 75 - 76 inches (for a 24 inch ball).
- Blow up the ball to where you think you are close to the proper size.
- Take the string and wrap it around the ball holding the ends together.
- Continue to fill the ball until it is tight against the string.
Hope this helps (o: