Stiffer the board the shorter you can get away with, when someone rides a board too small for their weight it basically makes the board a lot more flexible, for instace on my tiny park board whn my fat ass rides it, its like a chattering noodle and hard to hold carves, it'll go whump whump whump, and feel squirrly at high speed. However I can certainly ride it on pretty much anything, and turning it is super quick compared to my stiffer flex longer board.
So I guess theres a banalnce there do you want only high speed big mountain carver or something in the middle? Then you can cheat and get a really stiff board in a shorter length.
Anyways most companies have weight charts for their boards.
What I have noticed on my current board though, is that trying to hold a turn/edge when the terrain is choppy, is becoming difficult- and I don't know if that's just a function of the terrain, or if that is indicative of the board itself?
Could be the board, maybe its a flxier board, or you could be too heavy for it, because thats what happns when you ride a too flexy board on gnarly terrain.