Sir George's law states: F(drag) = 6*pi*a*coefficient of viscosity*velocity (it would greatly simplify the expanation if I could reproduce the formula for you.) The formula provides a solution to how fast a sphere will drop in a viscous fluid. A parachute would be considered a sphere and the air through which it's passing a viscous fluid. It's found that the speed is highly dependant on the radius of the sphere, hence the pi x a (a being a representation of radius) as well as the viscosity of the fluid, which is found by yet another formula, which I can't reproduce in this forum. The importance in your problem is determining the speed of parachute as it drops through the air which is directly related to its size... which is pretty intuitive, but Stoke's provides the mathmatical solution for you...