You can actually make a 2 car garage seem quite big. My first year was just a big open garage with black plastic stapled on the walls.
But then we went all out with hallways all through it. We use gaylord boxes (aka huge cardboard boxes used on pallets) hanging from the ceiling and garage door while in up position. I use zip ties to connect them together and installed hooks in the ceiling to hang them on.
I also move a lot of things to my back porch to get the most room out of the garage.
The last part of my yardhaunt will go through my detached garage that has a side entrance. To maximize space and allow for some good prop scare (and maybe one actor) I will utilize the exposed roof joists by hanging incredibly inexpensive light duty tarps from Harbor Freight that they seem to put on sale every other week. I am talking $3 for 7x9 (dangling from enough lightweight chain to allow the tarp to nearly touch the ground), so they are very good for walls as long as you don't actor or lunge prop scare guests into them. Am starting with a simple 30" hallway down one wall to add walking time, then turning into the rest of the garage (1.6 car widths worth?) that will be bisected in a sort of triangular fashion. If you look around most modern haunts, you will notice that triangles are definitely the best way to create the illusion of the garage being bigger than it is. The other benefits with using cheap tarps is that they have eyeholes already built into their corners for easy hanging, and they should be able to come down to be reused next year or for whatever. The negative, they are blue and reflective... similar to the ubiquitous black plastic liner wall ideas. But my garage is going with the clown thing, so blue works. And they make some slightly pricier (by $4 more each?) grey tarps too if blue is a dealkiller.
Check my most recent update to my haunt's blog to see an aerial pic... and maybe try to do the same snipping tool pic to then play with drawing lines where you imagine your walls to be. http://thekerbeyhouse.blogspot.com/