When thinking about designing a good trainer model, There a few things that you should take into consideration.
Having a high wing ( attached to the top of the fuselage) will help lower the center of gravity. Basically, this means the plane will "hang" from the wings. This will in turn make the make the plane less prone to roll. The bad side of this how ever is, it will make inverted flight a little tricky.
Dihedral is the upward angle of a fixed-wing aircraft's wings where they meet at the fuselage. This helps create a self righting or leveling effect on the plane. Meaning, after you have initiated a right or left bank using ailerons, the wings will force the plane to level out of the roll once the ailerons are returned to the neutral position.
Below is a basic illustration showing the measurements and proportions of a standard trainer plane.
I find it easier to start out with the dimensions of your wing, and go from that point to design and build the rest of your plane. Once you have the wing span, the chord, and the airfoil design, You can then start to get the rest of your planes measurements from that. You can also calculate the wing load of your wing to know how much weight it will lift, and figure out your power plant as well.
Here are a couple of links that I have used to help with calculating wing load, and power sizing.
Hope this is useful to some newer pilots out there trying to get their feet wet in electric rc flight.