The plans/Manufacturers manuals have recommendations for engine sizes to go with the models. If you're trying to design a plane, its a whoe new game. To determine deterimine if a plane will fly, you need to look at wing loading and drag. A low wing loaded plane will be easier to keep aloft with less engine (sail planes typically fall into the 5-7 oz/sq-foot range). A higher wing loading, lower drag configuration, as seen in many sport planes (15-20 oz/sq foot) will need more speed. Planes with even higher wing loading (20-24 oz/sq foot) need even more. Planes will fly with thrust/weight ratios less than 1, as long as the engine has sufficient power to overcome the drag on the wing at the required velocity to create sufficient lift.

Thrust/weight ratio is what determines vertical performance. If you want the plane to hover, you need a thrust/weight ratio greater than 1. You can get a rough estimate of the approximate thrust by using a calculator such as Thrust HP and use the Manufacturers rated horsepower (estimate about 75% of what the mfg claims). Look at different prop combinations, adjust RPMs to approximate power, and you'll get an estimate of thrust.

Brad