I'm looking to replace some old, Marvin single pane windows. I'd rather not tear off the trim to find the exact unit size. The sloped sill is always a problem.
Would a Marvin distributor have an old window size catalog?
If you don't want to remove the trim, it sounds like you are looking for a replacement window that will be installed into the existing frame. If that is the case you don't need to have the old size. You will just need to measure the existing frame.
If that is not the case, measure the daylight opening of the window and identify the type and approximate age, a Marvin dealer in your area should be able to determine the size.