What are the guidelines or rules of thumb you use to decide what size engine you'll use in your scratch-built scale projects? Obviously if you're working from plans, the plans give a recommended engine size.

It there some sort of wingspan (single or bi) to weight to engine size chart somewhere out in the world. And to what extent does a/c type influence you decision. For the smaller sized models the three major engine types seem to be the standard 2 and 4 strokes and RCV engines. I must say the shape of the RCV 90 seems awfully appealing for inline WWI types.

Also as long as I'm asking I'm wondering how people make weight estimates for their finished birds.