I don't completely understand all that is involved in the smaller tails on biplanes, but I believe you are correct.

First the the top wing interferes with the flow over the bottom wing, so you can probably consider the bottom wing to be 30 or 40% smaller. It seems to me that the tails are even small when you adjust for the wing efficiency, but this is a very unscientific opinion.

There are other aspects that may effect tail size selection. The tail on a biplane (or triplane) often has the air flow disturbed by extra wings. caban and interplane struts, and flying wires. The horizontal tail is not as effective at allowing the CG to be moved. Also drag is more of a factor in the performance of biplanes. I'm not sure that adding a huge tail to a biplane would give you the same handling changes that you get in a very clean and fast flying monoplane.

Also, as I recently learned, the early biplanes had very short noses (because of the heavy engines of the era). When we model these plane we tend to wind up with low pitch and yaw moments of inertia. This makes the elevator and rudder very effective.

I hope someone who understands the aerodynamics of biplanes, will comment, as I am interested in this issue, too.

As for designing, I'd look at planes that fly like the one you want to build. If you want an aerobat look at some Pitts or Ultimate models and see what they use for tail sizes etc, etc.

Carl