I do more or less the same as ShawnDriscoll.

At Conucopia3D you can get a "mesuring stick" object for free. I have a similar one I made myself (didn't know the free item). It's very handy when you import object that haven't been scaled properly. For instance, a house: If your in doubt about how tall etc. it should be, place the mesuring tool in a doorway and scale the house so the doorway is about 2 meters tall (give or take).
Of course you can scale things by comparing objects already present, but then you'll frequently have to make duplicates and depending on the nature of the objects, polygon count could skyrocket unnecessarily.
I could upload my version of the metric bars to the freestuff, if you'd be interested.