as said, it just depends.. a big buck could be a tall massive 6pt or it could start as a decent 18inch wide 8pt and up.
To me a big buck is any buck that is mature meaning 3.5 years or older.
Normally a shooter for me is anything with 8 pts and 18inches wide. But it could also be 8pts, narrow, but tall 12inch back tines. I have a thing for wide 20inch mature 6pts though. Ive got a few of those on the wall. Not your normal 12inch wide 6pt now:-P
now if I were out in the midwest in say Ohio then it would have to be a massive 8pt with tall tines. Or a 3.5 year old 10pt
over all its about 20-40inches difference in size on average between our bucks in Va and the midwest deer