We have these giant number plates as a standard size and must have the BSEU............. stamp on them to show they are legit. Colours also are required. The police enforce all this.
In the US I noticed some vehicles with tiny wee number plates, with lettering not much larger than an atom.
Also it appears you can have anything you want printed on the plate, for instance as you get near Las Vegas there are loads of variations of Elvis and others. Maybe if you have the money in the US you can have virtually anything you want as a plate.