A regulation softball measures 12 inches in circumference. Sometimes younger kids use a smaller size since they have difficulty with grip. By high school you should use the 12 inch.
A regulation high school softball is the same size as at other levels, a size 12. The ball can be different in construction, how tight it is wound, but it has to be a size 12.
11" is the standard size for fast pitch softball. Slow pitch players typically use either 11" or 12" sizes. The 12" size is the recommended for slowpitch.
