The 3D Warehouse is like the Wild West - a big free-for-all. Anyone can go in there and add whatever they want.
Here's a useful blog post by my friend Eric Schimelpfenig, a kitchen/bath designer and SketchUp expert. He discussed a problem we've all had - downloading a 3D Warehouse model that's HUGE (or too tiny), and you need to change its size to fit your model.
For example, he found a dinner plate that could crush not only the table, but the entire kitchen:
And a knife that could slice an entire house (or an entire neighborhood):
A simple use of the Scale tool solves the problem in most cases, and you can also use the Tape Measure for exact sizing.
Eric also lists a few kitchen/bath collections in the 3D Warehouse that are of high quality, so you won't get stuck with UFO-sized coffee cups!
Anyone can design anything in 3D! http://www.3dvinci.net/