I need to know the fitting size for the clutch master cyl...

I have a 90 B2200, and I need to know the fitting size of the clutch master cylinder to order the correct hose, and slave cyl for the Tremec TKO600 that will be going in soon....

You mean thread size?
It should be standard, being brass I'd expect it to be 3/8"BSP
If unsure, head to a wrecker yard and cut one off a wreck and take it along for good measure.
It wouldn't be BSPT since it is designed to pull the ferrule onto the seat, not seal itself.