There is no such thing as the "absolute" size of an object, but easiest way to keep the units in check is to scale them so they make sense in the metric system. This is because gravity is set to 9.82 by default, which is in m/s^2, so even though the units are unitless in a mathematical sense, it seems the developers did favor the SI system. Thus, for things to make physical sense, try to scale your models realistically. That is, if you have a model of a man, make the upright value of his localscale around 1.7-1.85 or so, since a man is typically between 1.70m and 1.85m. You'll find you have an easier time making sense of the physics that way.