is there a way of measuring the pot size without taking the heads off or whether its oversize

Nope, well unless you can measure the base of the barrel where it seats against the case and then try to find the OD of OE and aftermarket barrels to compare. Even then it may only give you a vague idea of the piston size.

thanks buddy tried to measure from the stud to the barrel ...... I wont know till I split for a freshen up cheers