wilwood brakes

Hey everyone, I installed a set of dynalite pro brakes on the front of my barracuda, still have rear drums. They are manual brakes and are still very stiff and do not work like they should for being 4 piston calipers on the front. I came to the conclusion that I need to upgrade master cylinders to push them, any ideas on what size bore I might need?
I did put the residual pressure valve in for the rear drums, and I do have a proportioning valve installed.


Thanks,
Brad