Do I uprate master cylinder??

[C.A.R.]

I believe the Master Cylinders are measured by their diameter, although whether this is internal bore or external overall size I don't know.

What I do know is that there were are generally 2 different sizes of master cylinder and it's the larger one you want.
I don't claim any of the below to be 100% accurate, but it's as much as I know!
MC Diameter =
19.0mm - Fitted to very early 1.4, 1.6 and 1.9D cars (probably phase 1's) without ABS
20.6mm - Fitted to early 1.4, 1.6, 1.8, 2.0 and 1.9TD cars with or without ABS
22.2mm - Fitted to virtually all cars and engines it would seem after the phase 2 (16v 1.8's/2.0's) came about, most also had ABS - this is probably the one you have!
23.8mm - Fitted to all GTi6's and then from what seems to be the late phase 3's (2000-on) no-matter what size engine was fitted (this is what late HDi's / XSi's have installed) - this is the largest MC and the one which you want!