If you have a 16:9 projector, then I'd at least go with a 16:9 screen. Given your width limit of 90" I'd select 89"x50". Your 4:3 videos will then display at 66.5x50.
Can I assume you've set your width based on your seating distance and projector characteristics (resolution)?
Can I assume you've set your width based on your seating distance and projector characteristics (resolution)?