The construction rules are fairly specific, but very fast(if you have the cheat sheet handy, you can stat out a car in about thirty seconds, and it won't take more than a minute to write those stats down on a record card). There is no WYSIWYG requirement whatsoever, so you can bash your car however you like and let the stats show something very different, or even play with unmodified cars if you don't yet know what you want to do with a given car.
There are scale suggestions, saying that vehicles of a given category should ideally be at least X long and Y wide, but that's about it. It makes sense, since the template-based movement and firing arcs mean that using a car that is a much smaller scale could give you a notable maneuverability advantage, and larger models will find themselves moving farther over time. Even then, those suggestions are just that - suggestions, and there's loads of leeway in them, too.