For chord...... Don't go less than 10% of the overall chord. They usually write that as .10C. And you don't need to go greater than 25% of the overall chord for barndoor type ailerons.

For span........ If you go with the barn door type ailerons that're a major portion of the wing chord, you don't need to use more than 40% of the half-span when they are .25C. So for example, if your wing had a 10" chord and 60" span, those ailerons would be 2.5" chord with 12" span.

If you go with strip ailerons that're 10% of the overall wing chord, on that same 10"x60" wing, the strip ailerons would be an inch wide and wouldn't need to go more than 18". However, most everyone usually goes full span for convenience.

Keep in mind that with a cambered wing, it's sensible to rig the ailerons with differential. The one going up moves about 2 to 2.5 times as much as the one going down.