One would think so but in reality it's the other way around. Make a fin too small and you get dutch roll where the model won't track well in level flight and often will have sort of a swinging oscillation from side to side. In really bad cases it'll swing so hard it'll induce a snap roll and spin. Marginally small shows up in the turns with a model that seems to want to hang nose high and shows signs of adverse yaw when using ailerons.

On the other hand if you make it too large the model actually becomes unstable the other way and wants to drop off a wing on either side and it becomes hard to control just as Stratman is finding. The size of the vertical fin really does fall into a smallish range of acceptable size for any particular design.

Factors influencing the size are side area in the fuselage, tail moment arm length, dihedral in the wing and center of gravity location.

A few years back I tried all this on a hack 2 meter aileron glider. I was dismayed at the control response of some of the sailplanes I'd had a chance to test fly and wanted to get a handle on all the factors. So I made a nice big all sheet fin and rudder for this model and went out with my TX and hobby saw in hand. I found that with the oversized vertical and a foward CG it was sort of controllable but showed some tendency to drop a wing in the turns. I move the CG back for the next flight and it was like trying to balance a broom handle end on. CONSTANT input was needed to keep it level. Out came the saw and off came about 10% of the vertical area. Next flight was about the same as the first with the CG forward. So off came another 5-8% and now I had a model that flew great in level flight and grooved very nicely in the turns up to about 50 or 60 degree banks at which point it wanted to try and steepen up again. A few more flights fine tuned the CG back a little and worsened the spiral dive bit so off came another two bits of fin on two flights. About 5% each time. The first made the model fly beautifully, the second made it start to show signs of adverse yaw despite the use of coupled ailerons and rudder. So on went the last piece taken off, a trim cap of monokote and I called it finished.

All of which goes to show that the vertical tail area is very important to how a model flies. There seems to be a "happy" range of about 10 % area between nasty on either side.