Clutch disc diameter?

Hi guys

I would like to order a new clutch disc and I would like to know its diameter before removing the gearbox.
FSM tells 9 1/4 (standard) and 9 1/2 (optional) disc diameter for slant-6 and A230 transmission.
How can I know the correct one for my transmission?
:thankyou:
Measure it Like Dave69 described or take it apart then order what you need.