The purpose of the horizontal is to restore the plane to a trimmed condition after a disturbance.
You want it to have as little drag as possible when the plane is trimmed.
Small horizontals have less drag than large....
And the lower the drag, the more power to be used to go faster.
To get the minimal stability needs, the small stabs go further aft; longer fuselage, more "wetted" area.
As the "disturbance" is almost always large, the 180 turn, there's no particular reason to worry about the drag increase then, it must be large, so size the horizontal for the level flight area.
Shape.. as mentioned the elliptical gives the optimum lift distribution for structural purposes.
A bit tougher to build though.
As with everything else in aircraft, any one part must be integrated/compromised with the whole package.
And in racing, it's mostly motor, then pilot, then airframe.