For 1/10th scale bodies, the "standard" wheelbase is 255mm, so that dimension is pretty much set. Otherwise, most 1/10th scale bodies are either 190mm or 200mm wide. These are not always the same, from front to rear, however, as many 200mm bodies are wider than that in the rear, by a few mm's. It really all depends on the make and model of the design.

As for total length, that will be up to you, as will height, but the aforementioned dimensions are what's used, presently. So, as long as you stick to the 255mm wheelbase dimension, all other dimensions are up to you.