A lot of folks ask what size board they need and others question if the board they have is too small or large. Most of the high performance manufacturers (I'm no longer going to call them small) can guide you on the size. Shops like Walzer and the WP can semi or custom fit you. Shred Stixx has the ability also.
Most hybrids os surfstyle boards have a rail that adds performance when turning. The Walzer, SS, WP, IS, etc all have shaped rails that work BEST when just the RIGHT amount of rail is in the water.
This picture is of Judy on a 4.5 Comp X. Amazingingly enough the board is too large for her. Most of the time when she is riding the rails are out of the water. You'll also notice the telltale sign of the spray coming off the inside rail...there is virtually none when Judy rides this board.