Is there a general rule or formula for determining engine size (or power) for a model. Is it determined by weight, wing loading, or some other factor?

Maybe there is a chart or something?

By what's written on the box or the plans etc.

I actually made a spreadsheet of the various options for my CG Ultimate. My criteria was overall weight and peak thrust. I wanted the lowest weight and the most thrust. Depending on which is more important to you, you may go toward a bigger or smaller engine. I also considered cost, fuel consumption, propeller size, and four vs. two stoke.

If you're just starting out with a trainer, use the recommended engine size or slightly bigger.