You would compute the wing area the same as any other airplane but need to say that it is for a biplane or a particular model and the reader kicks in the appropriate feeling that it is more or less effective that a monoplane.

For a model that seems like a lot of decalage (define here as the angular difference between the wing and tail). Remember that the decalage between the wing and tail is what gives the airplane angle of attack stability. Something that is highly desireable in the kind of flying the full size stagger bipe would likely be doing. In the model size it would tend to behave like a trainer with respect to pitch. It would need to have a forward CG and 1g flight could be achieved at a fairly low flight speed, but, at high speeds that angular difference would cause a lot of pitch up.

For a model that might get boring after a while when just cruising around and you get the urge to do an outside loop then a smaller amount of decalage would seem to be in order. Having never seen either the Morgan or Royal kit fly it is difficult to judge. Has the Morgan design actually been built/tested? You might need to ask the designer.