not necessarily... the size of the trackball refers to the overal diameter of the ball. Even a 3" trackball (depending on flavor/type highball, vs highlip, vs standard) doesn't expose ALL 3 inches above the CP.
What was the control panel from originally (that would help some)?
It's *probably* a 3" trackball as the other "normal" sized tracball diameter is 2 and 1/4" (for the ball itself, not the hole, mind you)
Hope that helps...
rampy