There are some VERY basic "rules of thumb" that produce good-flying RC models:

Choose wingspan, and make the wing with an AR of 1:6. Total aileron area = 10%-25% of wing area

Fuselage length = 80%-100% of wingspan span

spinner tip to LE of wing = 80%-100% of wing chord

Horizontal stabilizer/elevator area = 20%-25% of wing area with AR of 4-6. Elevators = 20%-35% of total horizonatal tail area.

Vertical fin/rudder area = 60%-75% of horizontal stabilizer/elevator area. Rudder area = 40%-50% of vertical tail area.

These are very basic numbers and will produce an airplane that will fly reasonably well. They won't give you an airplane that is good at everything. You can use almost any airfoil on the wing you want. Balance the wing at about 30% of the mean aerodynamic chord, and you'll be in the ballpark. Maybe 1 degree of positive incidence on a semi-symmetrical or symmetrical airfoil. Engine and stabilizer can be 0-0.

Draw these in realationship and put almost any shape of fuselage around them.

For a tail-dragger, just have the center of the wheels at or slightly-forward of the wing leading edge. For a tri-gear plane, the mains should have the wheel centers at about 40% of the mean chord back from the leading edge. You want the balance point forward of the center of the mains so that the nose will stay down on an empty tank. Too far back, and you'll have problems rotating for takeoff. Too far forward, and your model's nose either won't stay down, or it will tend to hop.

There you go. Very basic rules of thumb to start out with your first original designs. Once you have the model flying, you can then start to "play" with the parameters to alter how the airplane handles.

As always, there is a LOT of leeway in the parameters to get a good-flying airplane. In the 60's and 70's, many people who designed models for the model magazine construction articles used such guidelines to be able to "crank out" a lot of designs. People weren't so much interested in airplanes with particular performance as they were in having unique models they build themselves. With a bit of creative imagination, you can make a wide variety of "sorta scale" airplanes...mainly adjusting fuselage profile shapes, and wingtip and tail shapes.