A very short Google search lead me to this:
http://www.personalizedbikeplates.com/
Plates that range from the size of the ones that came out of cereal boxes to motorcycle/ATV and car-sized ones. They're plastic or aluminum (you choose) and they even have state plates.
So far, license plates remain a custom accessory rather than a required part on bicycles, but do you think motorists would react any differently if they saw a bicycle sporting a (non-registered) car-sized plate? Even though the idea to make them mandatory has always been dropped or rejected. But what if for the fun of it? Which is what these plates are for?
I wonder if they are even reflective, I couldn't tell looking at the site. I would put one on my bike just to see the reactions.