Not sure if this is going be be any help to you but at one point in time, I seem to remember one latex guru using width=\textwidth or \linewidth (or something close to that) to scale images when supporting multiple page sizes. It's been a few years so that may not be close to correct.
Edit: I did a quick Google search and came up with the following page:
Perfectly scale an image to the rest of a page with LaTeX which sounds as if it may work for you even if nearly a decade old.