I had to get nerdy and measure my plates, too. The ones I use most are 8". I got them at a restaurant supply store in NY. They're those classic diner plates with the maroon border. Not really anything pretty about them, but they're indestructible, and I guess they keep me from loading up!

I can't remember now because I got them a long time ago, but I bet I got them because they fit in my tiny NYC cabinet.