[mr0077]

The ratio between the master and the slave cylinder bores has a direct relation to pedal movement and effort. With no change to the slave or linkage geometry, a smaller master will increase the pedal travel for a given clutch movement and reduce the effort for that travel, and a larger master will reduce the pedal travel (for a given clutch movement) and increase the effort at the pedal.
You can change the master or slave to make the changes, but the ratio of the piston areas determines the relative movement ratios. Slave travel is dictated by the total system. If he isn't getting enough travel at the slave for the available pedal travel, he could either increase the master or reduce the slave.