proportions of a perspective-deformed rectangle

Question

Given a 2d picture of a rectangle distorted by perspective:

I know tha

Answer 1

Size isnt really needed, and neither are proportions. And knowing which side is up is kind of irrelevant considering he's using photos/scans of documents. I doubt hes going to scan the back sides of them.

"Corner intersection" is the method to correct perspective. This might be of help:

How to draw a Perspective-Correct Grid in 2D