Two scopes

So for right now I'm working on the 4.5" starter scope, after that I plan to make a larger (10-12") scope. The question is really how big a scope can I make that will still fit in the trunk of my car. It will be some time until I can afford the optics etc for this scope, so at this point its more a theoretical thing. For now I am planning on taking a tape measure to the trunk of the car.