maybe a rectangular one. Just just one. of three circle ones

or this

or this

none who wants to look at themselves eating? what would be reflected in the mirror?

I agree that it would be a little overwhelming to be watching yourself eating or talking in the mirror. However, I understand wanting to have a piece there to reflect light & give the illusion of a slightly larger space. Consider using a horizontal mirrored sculpture instead.

I'd think more about the style effect and mood you are trying to create, rather than just the size of the room. Obviously it needs to be to scale but it all depends on your personal taste. Would you like a sweet little antique-inspired Rococo mirror or a modern or transitional piece or....?