Heh, I know I know, I feel like a D student right now.
Wouldn't it be surface area though? And surface area is the 'size' of the target in the two dimensions we care about in this?
So if we want to create a square (just to make it really simple) of a certain size to be used at five yards, and have it look the same as another square at twenty yards....6x6" head box at 20 yards is 36 square inches. At five yards, we'd want it 1/4 the size (I'm saying that's the surface area though), so we'd want a 9 square inch area at five yards, which would be 3"x3". Not 1.5"x1.5", which would be 2.25 square inches and a lot smaller than scale. Tell me if I am completely wrong there. I feel like people had this discussion on PF a long time ago and people much better at math than I, said it was a trigonometry thing involving the type of stuff dove linked to in his post. This is part of why I err toward smaller targets for dry practice though and don't worry too much about getting the scaling exactly right.