Almost all OEM Toyota master's I've seen have the bore size cast into it somewhere. Alternatively, you can measure the bore size at the rear of the master when you remove it to adjust the booster push rod, etc.

Also don't forget when troubleshooting/adjusting your brakes that there's also a pedal pushrod adjustment on the backside of the booster.

'97 4Runner, '06 F350, '86 4Runner, '05 WR450