Like Chad said, 3/16 (or 8mm) is pretty standard for brake line
I like the biggest bore I can get on the brake m/c and the smallest for the clutch m/c. Makes the clutch feel easier to push. A 3/4 bore clutch m/c on a stage 3 or bigger press.plate will give you a big ol' left calf muscle
Yeah the fittings that screw DIRECTLY into most calipers are usually brass 1/8" NPT on the male end and whatever type of line you plan to run on the female end. I like to get standard 3/16 SAE double flare on the female end of the fitting because the european bubble flare is hard to reproduce if you want to cut the brake lines to the length you need and re-flare them. A $50 flaring tool will do great double flares on those nickel plated steel lines GIG mentioned. Just dont try to use one on stainless lines. Either the tool or your hand will break before you flare that stainless.