I was part of a joint effort to grind a 6" newt mirror some time ago, ditto what the other lads have said here Baz- there is enough worry and fretting and labor involved with even a small mirror surface let alone a medium to larger one, but each to their own and that sure would be a super challenge in the long term if it can be afforded... though the likelyhood of imperfections rises exponentially with each extra bit of mirror area i believe...
CS
steve