Since you are selecting neither a dress nor a suit, exact sizing is not a critical issue! However, some sense of proportion will make a statement about your tastes.
The higher the ceiling, the larger the ceiling medallion should be in order to maintain proportion. Ceilings are normally higher as the square footage in a room increases. Normally, the crown molding, furniture, lighting fixture and square footage will all increase as the ceiling height increases. Notice the crown molding in tall spaces. If it isn't larger on a high ceiling, the lack of proportion is obvious. Since most of our medallions are sold in specialty lighting showrooms, we've used the examples of an outdoor post lantern. A lantern 16" or 18" wide by 36" height looks absolutely enormous when viewed at eye level. However, when it is mounted on a post 12' above grade, it becomes proportionate in scale.
If your Arctel ceiling medallion is to be mounted in your dining room or bedroom, it should be approximately the same diameter as your chandelier, though it could be a little larger or smaller. The medallion's design should be the deciding factor. We've sometimes suggested a mathematical rule of thumb i.e. multiply the length of the room in feet x the width of the room in feet and divide by seven (7). This will yield the approximate diameter of your ceiling medallion in inches. This formula works pretty well on a 9' ceiling.
If the ceiling is higher, the ceiling medallion needs to be larger. Entry spaces or foyers are exceptions to this rule of thumb. Two- or three-story foyers should have a ceiling medallion that is generally much wider than the width of the hanging fixture.