The only problem you will find with smaller airplanes are that they are twitchy to control. I like to stick to wingspans of at least 36 inches or larger.
With the new micro receivers and servos that you can get today, I do not see why it could not be done. As long as you keep it light and pay attention to where you would be stressing the airframe why not?
Look at all those small foam RC airplanes out today I do not see why if you want to bash a balsa free flight to rc that it would be impossible....
Give it a shot!

EDIT
One suggestion I would like to make is Sig sells some 1/2 kits by Herr Eng. that are easily modified to 4 or 5 channel operation.
Rather than reinvent the wheel you might want to look at these. Also do some reading in the 1/2 a forum if you are interested in small planes. Or design your own go to a site called airfield models good advice to be found there.

The web address is:
http://www.airfieldmodles.com