Well, it's not really about your ceiling height. It's about how it looks over your buffet. If you choose something narrower, I think you could go taller. But really, a mirror about 2/3 to 3/4 the width of your buffet is what is recommended.

What are you reflecting in this mirror? Something beautiful? Will your dining guests be looking at themselves in this mirror while eating?

It's a bit about your ceiling height. You don't want skimpy/squatty. A 48 inch decorative round could be great, with the EMPHASIS on the frame versus the mirror itself, or 36 ish by 48 in a rectangle hung vertically. I'd not do less than 48. Allowing a bit of room at the bottom above the sideboard......you'll have about 41 inches above to the ceiling. You'll have room for lamps at the side, Don't worry about people looking at themselves. They love to look at themselves..........yes they do.