If you want to adjust the hydraulic ratio of your brakes to get better feel, then below is a list of possible donor bikes for which I know the master cylinder sizes. Remember, a larger master cylinder will give less power, but require less travel (potentially make the lever feel more wooden), and a smaller master cylinder will give more power, but require more travel (potentially make the lever feel spongy if you go too far). Some master cylinders have the piston size stamped on them (whether it be metric or fractional inch), but a lot don’t.
15.8mm – Very common, especially on 80s bikes, and modern bikes which have twin four piston calipers. Some specific bikes that I know of include: FZR250, FZR400, FZR600, SRX600. It seems to be almost the default size.
14mm – The VFR400 (NC30) uses a 14mm remote reservoir Nissin master cylinder. This is coupled to twin four piston calipers (unsure of the total piston area), and leads to excellent brakes.
12mm – The XT600 (90-2002 at least) uses an integrated reservoir 12mm master cylinder with a two piston sliding type caliper. This gives ok hydraulic performance (although hydraulics can only do so much given the wheel/disc size ratio), but feel is pretty shitty due to the bolt from the lever which acts on the master cylinder piston.
11mm – NSR150 has an integrated reservoir 11mm Nissin master cylinder. The lever is not adjustable for reach. I believe that it is coupled to a two piston sliding type caliper. Unsure of years, but the one I have has piston bore stamped on it.