I think width and surface area, especially in the tail, is the limiting factor. Without it you can't get the lift and leverage from the board to balance out the extra downward and sideways forces of the sail, even if you have a big fin. What you'll notice when sailing is that the board won't "break free" onto a plane very early and won't go upwind well. Also because of the altered geometry of the oversized sail, you'll feel like your body is forced into an awkward position no matter where you put the boom, harness lines, footstraps, etc.
That said, you should still try it just to see if you can make it work. Only 1 m2 larger than the recommended size might not be too bad.