ORIGINAL: daven

I have tested quite a bit of v-tail sizes on Q500 style planes and on a 500 square inch wing, a v-tail at 75 total square inches (15% of the wing) is about right. I have gone a bit smaller, and much larger. Any smaller and I was starting to lose too much stability in the turns for how I like to fly, and any larger just added additional weight and drag with neglible stability benefits.

With a 598 square inch wing, I would start with a v-tail around 90 square inches or each half at roughly 4 1/2" x 10. Elevators surfaces of about 1" in width and 6" in length would be plenty.

A v-tail angle of 110-115 degrees is pretty standard on most competitive Q500 birds.

It has been many years since a conventional tailed Q500 plane has won at the highest levels of racing. Q40 planes are predominately conventional tailed, but there has been a couple recent V-tailed models that are starting to take notice. The biggest issue I have seen with the V-tailed Q40's has been the take off, they have looked pretty squirlly. Not sure if that has been taken care of, but that wouldn't be a concern to someone looking at straight line speed.

Looks like you have a fun project going.