Planes advertised as "40 -size" are planes designed to fly on "40" sized engines. This means the engine has 0.4 cubic inches of displacement. (or 6.5 cc's). Trainer planes in this size are about 4 or 5 foot wingspan and weigh 5 or 6 pounds. a "60-size" plane is designed for motors (glow engines) of 0.6 cu inches displacment. Perhaps another foot larger and 6 or 7 pounds weight. a "1.20" plane / engine is targeting an engine with 1.2 inches displacement. These would typically weigh in around 8 - 10 pounds.

Somewhere around this size, the engine manufacturers begin to use the metric system. Especially gasoline engines. So you will see a 35 cc gasser. This is the same size as a 2.10 glow engine (about 16 cc per cu inch). And the planes in this size are often quoted as a percentage of their full-scale counterparts. So a 25% scale plane is one-fourth the size of the full sized plane.

For aerobatic planes such as Edge, Extra, Cap, a 35% plane would typically fly with a 100 cc engine and weight 28 pounds. A 40% scale would typically fly with a 150 cc motor and weigh 35 pounds.