The newer MCs (master cylinders) are roughly the same dimensions as the old one piece metal units, close enough that it doesn't seem to make a difference.
The Falcon guys like the 89-94 Geo Metro MC & booster combo because that whole assembly is less than 8" long and the booster is less than 7" diameter. Not only that but it's a bolt in with good pedal height and clears the mechanical clutch linkage on a stock stickshift.
For comparison a vintage Ford drum/drum or disc/drum MC is 7" and the SVO Mustang disc/disc MC is a hair over 8".
I, along with some of the Falcon guys, am wary of the Geo MCs ability to feed a 4xDisc setup, so I'm going to try and follow up on some of the work done by someone on TFFN.net. He ascertained that a Ford MC and the Geo booster will bolt together, so I'm left to figure out a couple things. I already found out the O-ring used on the back of the Geo MC is a non-issue, so I need to find out what the deal is with the MC pushrod, how good the pedal height really is, and how well it works with the Ford MC.
I'm pretty sure I can make the pushrod work, then it's just down to how well it works and the pedal feel/height, and I might end up relocating the pedal stud to improve pedal feel, since the 65-66 didn't have a separate PB pedal.
All that said, I'd like to hear about the Nissan booster, I can't seem to find a Geo booster assembly to test-fit no matter what I do.