Got an idea? Get it on paper; preferably graph paper. Here's a hint: most auto owner's manuals have a schematic of that vehicle with dimensions. Reduce/Enlarge on a printer can get your fit right.

If you plan to build an existing design (like a shuttle) you can use the assembly instructions for images and dimensions.

8 feet seemed proportional for the rooftop, but was also logical for efficient use of a 4' x 8' sheet of plywood.