Why does the size of a paper airplane affect flight?

With more surface area on the wings of the paper airplane there will be more air providing lift for the plane. If the mass of the plane is increased less than the surface area of the plane is increased, then the ratio of the force of gravity to force of lift should decrease, theoretically, allowing the plane to stay in the air longer.

There are other significant factors in how long the paper airplane will stay air born. The Launch speed is directly proportional to the amount of air moving over the wings which provides lift. However, the larger the plane and the lighter the material is the more likely the plane will deform at high launch speeds or from launching the plane by holding to far back on the plane during launch, which will make the plain more likely to drop from lack of lift if the plane deforms too much.