For a really well behaved, fully aerobatic bipe that won't bite you if you get a bit out of shape and too close to stall, I like a vertical tail with 15% of the wing area, and a horizontal tail with 20% of the wing area, for a normal tail moment in relation to wingspan. The 20% horizontal tail could be reduced to as little as 10% without too much nasty handling, but I would keep the vertical tail as large as possible. If the model is scale, just enlarging the vertical tail, while keeping the same shape is not all that detectable, appearance-wise, even with double the scale area, and your two-wing bird will be far easier to live with. Bipes do need larger tails in relation to their wing areas than monoplanes, due to all the turbulence stirred up by the extra wing and its cabane struts, open cockpits, and the like.