The smaller the bore size, the less effort is required for the same braking force, so you can apply more fluid pressure and brake harder.
However, a certain amount of fluid has to be displaced to bring the brake pads back into contact with the discs, which depends on how many ‘pots’ there are in the brake calipers, and their diameter.
The amount of travel of the lever is key, as you need a good safety margin between the lever and the bar, which reduces with a smaller master cylinder piston size. More lever movement is needed to displace the required amount of fluid.
A smaller master cylinder tends to give more feel as well, a larger cylinder makes the brakes seem ‘wooden’ and solid.
If you’re going to fit a smaller master cylinder, again make sure there is a good safety margin between the lever and the bars. Also make sure that the master cylinder piston retracts enough to allow fluid back into the reservoir, otherwise the brakes will lock on.
Modern motorcycle brakes are very powerful anyway, and capable of easily locking the front wheel on braking (although ABS will stop this happening), so it is probably only some old bikes or racers who would benefit from a smaller master cylinder.
Fitting a larger diameter master cylinder will likely adversely affect the braking.
Always check with a mechanic, if you have any concerns, as a front brake failure can be fatal.
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