measure The doors you have around your house or at a nearby garage .if you want to convert that to scale you can do math or buy a scale ruler usually found in the train department at the good old hobby shop.I would recommend going with two stalls vs three ,in most well executed dioramas less is more, meaning you should imply things and let the viewer fill in the blanks.It should tell a story.The exception would be a back drop for taking photos ,but there again you don't want the background to be too busy .there are a couple of good kits to start out a dio like the one described. fujimi makes a garage and tool kit ,estes made a 1/24th garage a few years ago and theres one more that I can't recall at the moment.craft stores like michaels carry diorama kits leaning more towards foliage, water, snow and other ground cover.try to plan as much as possible before starting .take pictures of aspects you want to incorporate. these are ust a few ideas none are set in stone use what you want but make sure you post some pics of your work I look forward to seeing it .Good Luck