If you considerably increase the area of the horizontal stab and elevator, you will have to move the CG back quite a bit, since the horizontal tail will carry more of the airplane's weight. Otherwise, the airplane will be nose heavy, and require a lot of additional elevator travel to perform maneuvers and to flair for landings.

Doubling the area of a typical horizontal tail would probably require moving the CG back from something like 33 percent of the wing chord to something approaching 50 percent, which will lower the effective wing loading and reduce stall speed significantly, and quite possibly, considerably improve tracking in maneuvers, although it might look a bit odd.

Many competition aerobatic models have horizontal tails with about 30 percent of the area of the wing, along with rather long tail moments, while the average sport model normally uses a horizontal tail of under 20 percent of the wing area. Elevator servo torque requirement will tend to increase somewhat, particularly if the elevators are given a rather wide chord.

Increasing both the span and chord would result in a more effective tail than would simply doubling its chord. Simply doubling the chord would result in a tail with a rather low aspect ratio with a poorer lift/drag ratio than that of a normal tail.