Bryan is right on the money on this issue. What is THE right size for a horizontal or vertical stab? You'll get different answers for different types of flying.

The stab and wing relationship is straight forward to determine. A stab that is about 1/3 the wing area makes a base for a great trainer. For a good aerobat that's just too stable and one wouldn't be happy with the overall flight envelope.

But the tail moment must be considered as well. Some smart folks in sailplane circles have devised a convenient calculation that considers the wing, stab, tail moment and wing MAC. It's a ratio of stab/wing areas X tail moment/MAC lengths. It's THE driving calculation for pitch stability and it has a name....Tail Volume Coefficient. Many sailplanes deal in TVCs less than 0.4. Their wings are more efficient than everything we fly in pattern so going that low on TVC makes for very efficient flight.

For pattern, I've seen TVC greater than 1 for some planes, meaning much too stable in pitch for efficient aerobatics. Great for trainers though. My designs tend to hover at TVC closer to 0.6, making for very spritely performance around the whole envelope.

Lately, the trend has been to smaller wing areas with no adjustment to stab area. Guess what...these are more stable in pitch and are actually countering the very thing the "designers" are trying to do. The results will work but at lower efficiency of control command. This simply means that servos work harder to achieve the desired goal. Yeah I know some will argue "I love the way my XYZ snaps". I can only say fine.

Similar calculation can be done for the vertical ratio of areas and moments...it's just trickier because a fuse gets in the way. You need to do some estimation of where the nose area ends and tail area starts. If you want to make rudder more effective don't automatically add rudder area but consider fuse area increase around mid ships. OR do as Nat did and add a dorsal and ventral strake at midships. Also addition of a smaller dorsal just in front of the fin improves the rudder's ability to pivot the tail in yaw. It tends to concentrate the center of pressure (in yaw) closer to the fuse center line. There is almost no moment to speak of vertically so there is minimal adverse effect to this addition

Folks, Nature has examples of dorsals fins all over, just take a closer look. Take the dorsal fin off a shark and see how the fish turns....

Understanding the Vertical and horizontal TVC is fundamental to what we do in pattern. That's a great place to start a design. There's an awful lot more of course but the rest of the details are trimmings to the tree

MattK