I just found this, I don't remember if I answered you. A 51" is not the only thing that matter, The width is just as important, I would go with a board at least 20'' wide. I just like lots of planing surface without the board being long. I guess that's why I like the fish shape.
i saw your post on the "board size help " thread and i was wondering if a 51" board would be good for a 5'5 140lb kid like me. ?