Generally 4 x 32mm pistons would need 19mm bore master cylinder but it depends on what feel you want. I like lots of feel with the minimum of lever movement. 19mm Bore with the 32mm GP4RX is lovely. My mate loves what I would describe as a wooden lever feel. Another mate wants lots of lever travel, so uses a 17mm. So if you go down on M/C bore size you get more lever travel and feel, up in bore size less lever movement and a more wooden feel. Then you get into pads. Lots of initial bite or a softer initial bite. I like softer initial bite so it doesn't upset the bike, then the more I squeeze the lever, the more power I want. Try the std master cylinder first. Only takes bleeding to try it.