The ceiling medallion should not be the same width of your fixture, it should be bigger or smaller...But the hole in the medallion should be the same as the canopy, which is in your case 5 1/4".

I was doing a tray ceiling design for a customer a while back so we were going to build him a drop for the light in the center... when I use to work for an old school carpenter, he would take the room width x length /7 = medallion diameter ... so if you have a room 12' x 14' / 7 the medallion or what ever you put up there, the overall diameter should be 24" I don't know where the 7 came from and I don't remember if he explained to me or not, but I been using that calculation all the time and it seems to work as proportions go.

With that said, the general rule is, the higher the ceiling the larger the medallion. Medallion should always be proportional with the space you hang the light... so for instance if you have a dining/living room combination and its all one big room, medallion should be proportional with dining room area only.

Good luck, I hope it helps.

Ahh, I love ceiling medallion catalogues! The best Houzz p+rn!! Great fun!

Thank you for your help with this GN Builders L.L.C. I have one more question for you (thank you for your patience - I'm such a newbee). If the ceiling has an orange-peel texture, should I apply the same texture to the medallion before I paint it to give the seamless look, or would that ruin the molded effect?

IMO it don't have to be a match, the medallion considered to be a decorative ceiling component, so it can pick up a different shade or color to stand out and catch the eye.

With that said I'm not the decorator type of a guy, I design and build things but when it comes to decorations I'm with you in the same boat :-)... In addition, just try a few things and have fun with it, because no matter what the outcome will be, if you like it, everyone else will because it was meant to be that way...
In the mean time, maybe someone here with more experience can help you out with the color coordination or as decorating goes to dress it all up.

Good luck

If you are referring to a hole in the middle of the medallion, the standard size opening is 4".
It is to accommodate easy access to your electrical box. I am attaching some of my work with 4" opening. Also, a general old rule of proportions is a safest way, but not always the best way. You have to look at your space as a whole. Sometimes, a larger medallion will work much better with your space. The problem is, almost everything that comes in one piece today, is so thick, that it looks very heavy, bulky, and out of place. But that is the topic of a different discussion :-)

If one has a 5" canopy or as this poster, hers is 5.25" would the medallion inner hole be ok with a 4" hole? Then the canopy will go over approx 1 inch diameter over medallion or is that what we want? I was told to cut the hole to match exact size of canopy but no way do do a nice job and make it smooth?