Once you have figured out what scale you what to build the best thing to do is make a mock-up of it, this will save you from wasting money in the future & makes sure that what you are building is to the scale & size proportions to what you want to display with it.
When you have the building in your head of what you want to build use your scale ruler to draw & cut out the patterns to make the basic structure. In this case the side & end walls, & the roof. You can just draw the door or window openings in at this time you are just making sure this is the size you want when the project is finished before you put a lot of time & energy into the build & it’s not the size you had in mind when finished.
Once the pieces are cut out, lay them out flat, tape them together, then stand them up & tape together. Now you may want to sit your scale vehicle next to the building to see if the overall dimensions, door placement, & roof overhangs, etc. work for what you have intended, if not adjust it to your liking.
If you are building a diorama with many different structures making these mock-ups first can really help you visualize what your end product may look like. This also saves you a lot of time, by not having to rebuild a structure because the size doesn’t work with what you originally had in mind.