If you can tell me what model the motors are, I could give you a good ballpark for what the 3S battery will do with various props using Drivecalc (assuming their data is in the database).

BTW, yeah, that is typically how I would do it. I wouldn't necessarily pick the largest prop, but one of nominal size. You wouldn't put a 4-6 inch prop on a 40 sized trainer, and you wouldn't put a 12 inch prop on your funjet. I figure this is the easiest way to start looking at combos.
Curtis