I am working on my 27% Extra 300S.
I would like to enlarge the Elevator and Rudder for more agressive aerobatics and 3D.
Should I add counterbalaces to the elevators or keep the straight hinge and enlarge only?
Is there a certain equation I should follow?
Thanks

I would think you may want to look at the counterbalance method first. That will keep the amount of force lower on the servo while giving you more bite to the airflow. Another thing you may want to consider is to beef up the rear part of the Horizontal Stab that you hinge from, there will be a lot more stress there; caused by enlarging the tail feathers.
Now as far as ratio's I am not sure but one thing you may want to consider is the ratio of the horizontal stab area along with elevator to the main wing area. the more you change it the more stable/unstable it might become.

The easy option is to just move the hinge line foward, keeping the same total area, on both fin and tailplane.
evan.

That would be a very good option! By far the easiest to boot.

I was thinking he could make it a flying stabilizer, but it might be a bit squirrelly.

OK thanks guys.
I am not radically changing it.So maybe I'll just move the hinge line forward and or add a bit to the elevators.
3D flight was not really a household word when this kit was popular,and maybe I should not try to turn this aircraft into something it never was.I'll tweak it a bit to improve responsivness at slower aerobatics.