What sort of stuff do you want in this “backdrop shop”? You should be able to find plenty of things to use in the 3D Warehouse to make your own. Usually when I want to provide some sort of scale reference for a furniture model, I just use a 2D human figure.
How would you use this virtual ruler you’re asking about. You should be modeling at fill size. For 2D illustrations you can add appropriate dimensions in LayOut. If you want some sort of scale to use in LayOut for picking dimensions from, that’s easily model in SketchUp or drawn using the Scaled Drawing option in LayOut.