I would look at a site like Fischer and Jirouche or Felber Ornamental Plaster for a genuine medallion in an appropriate reproduction. Victorian medallions tended to be smaller in diameter, or rectilinear/angular shapes, but chunkier and more dimensional than earlier medallions.

I'd start with the ceiling color first. One of the functional reasons for them was to disguise soot from the ceiling fixture.

Agree w/all April’s comments. . Formula is a good guideline but your Chandy is kinda longish. If you use the math and your dining room is large, the outcome might be a larger medallion, which might look a little off with a longish Chandy. So take that into account and make sure it’s pleasing to your eye and not strictly a math decision.

i think the medallion shouldn’t be much bigger than the widest part of Chandy. I think this is nicely sized

this is a fancy double medallion. I love it but when I’ve selected mine, I’ve opted for simpler ones, not to outshine the Chandy itself

The above medallion is Greek Revival Late Classical period and the central portion is almost identical to that in my former front hallway.

I would look for something heavier and more geometric.

I have ceiling medallions on a few chandeliers in my house and love them (light pain ceilings, and white dove medallions to match my moldings). However, I think it might be too busy with your different color walls, wallpaper border and curtains. Are you going to paint the walls and ceilings or keep as is? It doesn't look like you have any white or off white in the room. Perhaps a shade similar to the metal on the chandelier?

Felber CM-011, CM-020, CM-019 (smallest); and CM-030 and CM-031 without their center rosettes and the more Victorian style medallions

Your chandy is gorgeous! I'm sure you're an expert on the era of your house and perhaps have found photos of medallions.

Note that mine are very shallow, not thick, like many of the ones I've seen for sale to be applied.

There was a modern, dark shade fixture in my 1939 dining room that everyone hated. When buying a chandelier, I went with eclectic --- and wanted to play up the "spider" web of my medallion.

The photo makes my chandelier look bigger, but there's a few inches of medallion on all sides. As mentioned in that thread -- mine is plaster, like the ceiling.

My ceiling is only 8' high and the room is 12x12.

This one is in the foyer. There was a horrible light up there, again an 8' plaster ceiling. This fixture was a last-minute -- must do something decision.

By the Edwardian period, medallions were pretty passe because of electricity. There may have been something vestigial but it was "modern" to not have plaster cornices or other ornament on the ceiling.

I'm not strict in keeping with the Edwardian or any era per se, that is however a favorite era of mine. Not looking to create an authentic look, my house used to be a one room circa 1870/1880 school so if I were I'd have to take it back to open concept with desks and a chalk board.

Some thought will have to go into the style of the medallion as someone pointed out. I'm sure I'm going to have some tough choices. Thanks all.