That's up to custom sizing really. I have a Triathelon frame that is the largest frame in that companies build. It's 64 cm seat, 64 toptube.
I'm not even sure that it would fit you.
It's not mtb, more xc. Most frame builder did weld up frames for big guys, and these frames are useless to us smaller builds so they CAN be found.
But they were expensive (custom) and not produced often.
Tom Ritchey did a lot of frames for big athletes during the late 80's early 90's.
These come up on ebay once in a while.