A question about accurate UV mapping

I was wanting to know if it were possible to know the relative dimensions of a selection of faces. What I mean by this let’s say that I have an image composed in Photoshop that 16:9 or 4:3 or any other bias. Let’s say I’ve made a selection on a cylinder, in this case a beer bottle. If I knew the relative dimensions of the selection I believe I could accurately apply the image as a UV map and in theory there should be little or no distortions of the image. Is it possible to determine the relative dimensions within blender?

You can see the pixel coordinates of any selected vertex or 2d curser.