Once you have your view size chosen (ie. the first view is drawn), you can base all your pictures off that. I'd look at Police Quest's locker room, and the size of the views in it. You'll get a general idea of proportion.
Just remember, it doesn't need to be perfect. As early Sierra games don't support view scaling, there were many instances in Sierra's games where the view seemed to "grow" in propotion to the background as it moved up the screen due to the picture's perspective. It's no big deal.
Good luck!
