I am a nerd, I had to measure my Fiestaware. (Made in the USA, which is the main reason why I bought it. That and it's pretty darn tough and cute.)
The dinner plate is 10 1/2 inches across. The part where food can actually sit is smaller than that, of course. Looking at how they fit in my cupboard, I don't think 12 inch plates would fit in the cupboard at all.
Wanna freak yourself out? Watch the Sesame Street clips from the first couple seasons (60's). Man, everyone was so skinny back then!
"If Americans want to live the American Dream, they should go to Denmark." - Richard Wilkinson