In fact, by using logic you can answer a lot of questions concerning proportion without even looking at a full scale tree. We use a proportion or ratio of one to six for trunk diameter and height respectfully to achieve dramatic proportions in bonsai. A twelve inch high bonsai should therefore have a basal trunk diameter of two inches. Because of this deviation the resultant bonsai has even more impact than a full scale tree, and unlike a locomotive fits nicely in a corner of the living room. It's convenient, however, to have a mean or guideline to go by.
Cars are longer than they are wide.
We'll conclude with a return to the subject of scale. Somewhere it says a bonsai shouldn't be any taller than 48 inches. I don't know who first said this, but it's why all of my bonsai are under 48 inches. He may be watching me. But for all purposes of discussion let's say that 48 inches is as tall as tree models can be and still be regarded as bonsai.