The size of the wing is arrived at by first specifying a desired landing distance. This in turn — assuming you will be using conventional brakes — implies a maximum landing speed. Now, the lifting ability of a given wing at a given speed is limited by certain physical laws. You can raise the limit by adding big, complicated flaps, like the marvelous ones that emerged from the wings of the Boeing 727, but they add to the weight, complexity, cost and difficulty of construction. So you first make a tentative decision about how sophisticated your wing is going to be, and that leads, by way of the maximum lift coefficient, which you look up in a book, to a wing loading. Dividing the gross weight by the wing loading gives you the wing area.