My bike originally came wih an ATE caliper, and replaced with a Brembo unit from a a wrecked R65. I measured this more than five times due to the discrepancies found online, thus I am pretty sure the R65 equipped Brembo calipers have 36mm pistons. Look at my post about calipers at the FAQ section.
I also have a brembo caliper from a wrecked R100 and the piston measures 38mm. The larger displacement airheads use calipers made by brembo but uses a slightly different shell from the generic F08 shell, hence incompatible with R65 fork lowers.
It would be interesting if someone could obtain a 38mm F08 from older lambreta's or ducati's and install them on a R65.