if by fin you mean the vertical stab, make it look nice,, no offset and be done,,
if you mean the horizantal , make it 25 % of the wing area, split 60% stab and 40% elevators and that will get you in the ballpark.
if you make the motor bearers,, "maple rails" to all the way back to the wing leading edge, then its an easy matter to shorten the nose once its assembled by hacking it off, you can lenghten the nose by moving the motor forward and then filling in the empty space to the spinner with light wood. That way you can assemble the plane, check the balance, and adjust as needed... Just a thought..
make it simple, straight, and rigid,, but dont be a dumby like me and make it pretty , well not to pretty anyway,, you will want to be able to fix it easily..
the Twister fancherized is a great start place , it survives well, easy to repair, andiwth the Fancher mods it a great platform. Build two or three, or go with the Skyray 35 and do the same thing, but the Skyray will want a 25 or 20,,