Here is a link that will answer some of your questions. Someone else was kind enough to give it to me when I was asking these same questions.

Here is a link that might be useful: link

Thanks, Brutuses!!!!

Of course the "best" ceiling medallions would be true plaster ones, but those are expensive and difficult to install.

I put up a couple of foam medallions because the canopies on antique fixtures are so small and didn't really cover the electrical boxes completely. I wanted them to be simple and not really stand out. I found that often the recommended size based on room size sort of conflicts with the fixture size too. I used a 16" medallion with my 24" fixture because I did not want something that would overwhelm the fixture.

I ordered mine from architectural depot and they are made by fypon. They were inexpensive because they were small and simple and I attached them to the ceiling with caulk, making sure to caulk around the edges for a more seamless look. If I ever want to take them down, it won't destroy the plaster to do so. I then painted them the same flat paint as my ceiling so they had more of that "been there forever/part of the ceiling" look that I was going for. I painted several coats on them before hanging and then two more coats once hanging to more "seal" them to the ceiling.

Here's my LR medallion and chandelier:

And in the dining room: