It is typical to measure such things with gauges. Likely, a conical gage in this case, based on the spherical diameter of the ball end. From there, it is what the designer says it is, i.e. on the print. Both dimensions and gauging specifications are always specified on a proper print. The gauging may be an in-house or industry specification, though, necessitating further digging.
I realize that this maybe perfectly correct, and perfectly useless to the OP's specific question.
So what is the diameter of the spherical contact anyway?