How do I calculate the tail size of my rc plane?
There's a formula for calculating minimum tail volume coefficient (tail area X distance from the wing) for light aircraft.  Not sure if the same value applies to RC but it's a good starting point. This is from David B Thurston's book Design for Flying (Thurston was an engineer on numerous Grumman programs going back to the Hellcat).

(Tail Arm ÷ main wing MAC) X (Tail surface area ÷ wing area) = .55 or greater

Tail Arm is the distance from the C of G to the 30% chord point of the horizontal tail.

.55 minimum is for a symmetrical airfoil section tail typical of light planes like Cessna 150s.  I would use a higher number for a flat sheet tail surface, and being RC, so maybe .6 to .8.

While you're at it, Google "tail volume RC aircraft" and you'll find some more information specific to RC that'll be helpful.