IMO Any sized plate would not look good on the front end of a beautiful C6. You'd still run into the problem of how it attaches to the car without causing damage, prohibiting air flow, destoying aesthetics, etc.
First off, I don't think you can get duplicate plate. Meaning that if you had a car plate that was 12345, you couldn't get a motorcycle plate that was 12345, and vice-versa. Doesn't matter the vehicle, (I don't think) the state allow plates if they are already registered to another vehicle.
Second, if it is registered to a motorcycle, then it can't be on a car. Your car isn't a motorcycle.
Third, only plates created and dispensed by the state are legal plates. Even if you made your own exact copy but smaller, it's not the real plate, thus not legal to used as such.
I would imagine that it would be illegal anywhere. If it were legal I would assume that they would make the plates the size of motorcyle plates to begin with. Also there is a reason they are called motorcycle plates and car plates.
Just a bit of trivia about license plates: North American license plate size was standardized for the 1956 plates to all be 12" X 6". Before that, each state or country chose the size and shape it wanted. Used to have more interesting plates then, too! For example, Tennessee and Kansas plates were in the shape of their state. Other states, such as Illinois, had different length plates depending upon how many numbers needed to fit. A current exception is the NW Territory of Canada which issues a polar bear shaped plate, both for cars and a smaller version for motorcycles.
So I found this at the California DMV website
"Article 7. License Plates
4852. (a) License plates issued for motor vehicles, other
than motorcycles, shall be rectangular in shape, 12 inches in
length and six inches in width. The number and letter
characters on the plates shall have a minimum height of two
and three-quarter inches, a minimum width of one and onequarter
inches, and a minimum spacing between characters
of five-sixteenths of an inch.
(b) Motorcycle license plates shall measure seven inches
in length and four inches in width, and the characters on the
plates shall have a minimum height of one and one-half
inches and a minimum width of nine-sixteenths inches, and
shall have a minimum spacing between characters of threesixteenths
of an inch.
Amended Sec. 1, Ch. 859, Stats. 2000. Effective January 1, 2001."
Pretty boring read 945 pages, only picture was of Arnie and I have no idea what moive its from.