Here's some although late, but still good advice. If you're looking for a reference - go to some local garages and take pictures of the cars outside. Or look some up online to get a sense of scale. It depends on what you're looking for in a model diorama. Are you looking for something simple, is it a two car garage, multiple cars, or one story with a sign outside? Do you want a set of pumps out front like a full service station, or just cars outside? Is there a yard for customer cars waiting for pickup, or repair? The choice is yours. Use an airbrush if you want the best results. I use TRUE NITROCELLULOSE LACQUER - NOT ACRYLIC (as acrylic is NOT lacquer) for the ulitmate in glossiness, and ease of use. It's easy enough to wet-sand to get a smooth finish - I wouldn't use anything less than 1500 grit for this, and use Meguiars" No. 7 polish by hand to get that true show car look. Do this with an old CLEAN T-shirt, or with a terry cloth towel, or wash cloth, or for small hard to reach places a cotton swab.
If you need a general guide for measuring cars here's a rule of thumb that I use: if you're using any scale remember that the space for a vehicle is that a garage door is only a little wider than a car. Meaning that if this is 1/25th scale the door is going to be 3/4" of an inch wider on each side. The top of the door is going to be 3" - 4" inches taller than the top of the car. This is typical of most garages as there would be more room at the top for taller vehicles. If you're going to build 1/18th scale the same distance is going to be the same - 2" 1/2 inches from the side of the body to edge of the door, and 6" - 8" inches from the top of the car if you want to use the scale that is correct for the scale of car that I've described here. If you're using trucks then it'll be a larger hole. Use this as a key for determining what your needs are. Usually the vehicle will be 1/3 of the size the hole for the door will be, except semis. Those will about 3/4 of the height of the door way. The same with the width: the door will be at least the same distance - about 10" - 11" inches wide for 1/12th scale cars: 6" inches wide for 1/25th scale, and 4" 3/4 inches wide for 1/24th scale cars. Four to five inches tall for 1/25th scale cars. 3"1/2 to 4" 1/2 for 1/24th scale cars, and eight inches tall for 1/12th scale cars. This is a general rule of thumb that I go by for building a PROFESSIONAL garage that repairs cars. It's accurate, and easy to get the correct sense of realism. I hope that this can help in the future if I'm too late to help on this build.
~ Cobra Chris