A rough rule of thumb is to have the tail moment arm around 2.5 or 3 times the MeanAerodynamicChord. And the HorizontalTailArea about 20% of the WingArea.

Since you plan to use an existing TMA and your WA already exists, you could simply plug your existing measurements into a formula like the one I used last time I drew up a glider.

HTA = (3 x MAC x .20 x WA) / TailMomentArm

That would tell you what area would work with about the same authority as a 20% area works in the average layout.