I recommend somewhere in the 20% to 25% of the wing area. I have flown kits with a 30% horizontal area. There is really no down side to a large tail. It gives you a wide lattitude in CG placement.

The vertical area is normally 1/3rd to 1/2 of the horizontal area. There is a down side here. Too small and you get into yaw instability. Too large and you get into spiral instability.