Your best approach might be to take a photo(s) of your buffet and blank wall and then print proportionate mirrors from various web sites. Overlay your buffet wall with different sizes and styles of mirrors until you find what pleases you. I'm thinking that a large (~25" diameter) mirror might look best but that recommendation depends on so many things, i.e. height of buffet and ceiling, other wall decorations, wall covering, etc.

Big.
The bigger, the better.

I mean bigger mirror area, not just heavy in the framing, so that it opens the wall visually, and can reflect lots of light.

Big enough so that you can hang it with the bottom edge fairly close to the buffet surface, and have it nicely framing the image of a tall person in the room when he or she is standing up.