You also get higher pressure at the other end if you push harder with your foot.

The problem with small diameter is you have to press the pedal farther to activate the brakes. That means it takes longer to react.

It's all a tradeoff.

isnt smaller better? as you get a higher pressure at the other end

Bigger is better, but harder to push down on - you get quicker acting brakes and a firmer pedal.

Also if you are not using a proportioning valve (like most auto manufacturers) then the relative sizes of cylinders and brakes "balance" the braking system. So using a different master cylinder causes the balance to be off.

7/16 is a really small master! ... im only running the s13 fronts and drum rear so we will see how it goes ... might try and get the 7/8 master. the car also has non-power assisted brakes

I have done some asking around as I am going to run S13 front disks and calipers and R31 rear.
they've said to use a 7/16 master.

You will need about a 7/8 inch master, and will have to use an adjustable bias valve in the rear line.

bigger for sure. as piggy dog is trying some master cylinders, it turned out that s13 master lock up the back on any type of hard breaking he is now going to try r33 and will se what he says.

im about to go the s13 brake upgrade but want to know wether a 11/16 master will be enough. i got a new one in at the local supplier but forgot to ask for the bigger one lol. will this be enough or will i need bigger?