What Dipstick said is pretty much how you could do it here with a 99% guarantee of success. The real equations for horizontal and vertical stabs are normally based on amount of drag, moment of inertia and lift. Too complicated for the "fun" part of this hobby. But here is a little bit of info that could help you. For a long tailed model such as gliders, you can get away with a smaller size stab than a short tail model. Its just a matter of balancing the moments.