For my style of riding I prefer a board that is the height in between my chin and mouth.  At this length (160 cm for me), I get enough effective edge for the board to hold an edge while carving at high speeds, but its short enough to be able to still do all of my 'buttering' and freestyle tricks.

That's my sweet spot for what I do.  If you want more edge hold during carving, go longer.  If you want more manuverablility of the board, go shorter.