very difficult to find this as no one seems to have studied the subject or simply don't care.
The piston size is 14mm and the same master cylinder is used for the dual caliper and even the dual piston dual caliper setups.
14mm on single piston single caliper setup like the stock 80 cb750c simply sucks in both feel and hand lever effort.
here's a chart for further reference if anyone else comes down this road looking for updated braking.
http://www.vintagebrake.com/mastercylinder.htmsingle piston calipers are 43mm
blogg for upgrading the calipers