A 1/6th scale P51 might require a .46 cu in size engine whereas a 1/6 scale Piper Cub might require an .15 cu in. Usually a kit will have both designations as well as wingspan, length, wing area and wing loading. After you have been in the hobby for a while all you need is a couple of those pieces of information and if you are familliar with the airplane style you can guess at the rest pretty well.