Power system based on model size.

There is really no other way that i know of except to go with the manufacturers recommendation.
For example, if your model weighs 40 inches, then go with a glow/gas engine that is recommended for a 30-50 inch model.

Here is the table for Wing Span to Weight table for models.

as you can notice , as the model weight grows heavier and the wingspan goes smaller the chances for flying grows thinner and the model will experience difficulties to fly easily and may not fly at all.

Also as the wingspan grows larges and the model become lighter, the chances for the plane to be airborne gets higher. also the plane will have excellent flying characteristics and will float in the air like a dry leaf.

Note: the given figures in the above table are approximate and may be affected by various factors like the thrust of the engine, wind speed while flying, elevation from sea level, material used for building and so on. also this is not a strict guide and should be followed as such and small tolerances are allowed.