Are you referring to the big nut that retains the piston and spring assembly? I believe it's 1 3/8". A flat-ended punch against a corner of the nut and a vigorous whack with a BFH is the way we loosened that nut back when I was making a living working on them. We virtually never removed a master cylinder to rebuild it, just change out the internals with the M/C mounted on the frame!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!