To add to what Stu mentioned, you've got your Metric platters and your Imperial platters. Metric platters seem bigger but they use smaller increments, and with the whole Dollar/Euro exchange thing, it works out to about 10.27 inches per RPM, give or take a centimeter or two.
[I'm just here to help]
Seriously, Stu's right. I don't think there are any hard and fast rules, but a dinner plate is probably as good of over/under gauge as anything else. As far as ideas for usable sizes, poke around in your kitchen, ask your wife, check out the housewares department at your favorite store, and that would likely give you a feel for what's common.
Or, I could just toss out an arbitrary list and let everyone debate the numbers. That'll be more interesting.
Saucer: 4 inches or less
Change/Jewelry Tray: 4 to 6 inches
Plate: 6 to 10 inches
Platter: 10 to 18 inches
Tabletop/Snow Sled: 18 to 24 inches
Satellite Dish: 24 inches and up