it is going to be about balance (actually moment i think), and straight flight - there is probably a ratio of length/weight/diameter to work on too. - do you get to pick the launch angle? can it "glide" after launch?
i would guess as small as possible, slightly nose heavy, with three fins for directional control. if it is allowed to glide, i'd get a model airplane kit (inexpensive wood ones) and mount it, with the center of gravity, in the center of the wings (?) then experiment with that..
any add'l details would help. alternately, are you looking to build up more air (pressure/volume/velocity) for greater initial velocity?