It is kinda based on engine size and weight a 10 -15 size plane will weigh around three pounds with a wing span up to 48 inches, while a .40 size will weigh up to 5 lbs with a span up to 60 inches, .60-.90 from 6 to 10 pounds. There are also many factors involved, wing design, airplane construction. Take a look at the Peace-O-Cake and the Butterfly Huge wingspans but they take a .049 and a .15. Some of the Warbirds that have a 60 inch span might take a 90 to 120. It will also depend on what class of airplane, whether it is a Sport model, or a 3D, or scale. Today's 3D airplanes are very light weight and are powered to have unlimeted vertical performance, sport planes like a goldberg Falcon or Great Planes Sportster are not.

Back to the main question when purchasing retracts take engine size and weight into consideration.