A Major League baseball must measure 9 to 9¼ inches in circumference under MLB Rule 3.01. That is 22.86 to 23.50 cm around. The rule specifies circumference, not diameter.
Assuming a spherical ball, diameter = circumference ÷ π. This gives a calculated diameter of approximately 2.86 to 2.94 inches, or 7.27 to 7.47 cm. Real balls can be slightly out of round, so this is a derived range, not a separate MLB diameter specification.
The Official Baseball Rules also specify a regulation ball's weight range. Youth and other leagues may use different equipment rules; check the applicable rulebook.