I have a formula for board size. It's at http://www.vims.edu/general/sailpadd...Calculator.xls
under the Board Size tab. You just enter your weight and it returns 3 board sizes; 1) Your minimum size beginner board (for you it's 234 liters), 2) Your ideal first shortboard (for you it's 170 liters), and the minimum size board that you will be able to uphaul with practice (for you it's 139 liters).
Hope that will help.