I'd really prefer a taller piece of art to give a vertical emphasis above the horizontal nature of the buffet. But if you're absolutely in love with this piece, you should place it about 6" above the buffet, or if there is something above the middle of the buffet such as a bowl or some such, then higher. However if you have a high ceiling, you'll really will need taller art.

You're right that the dining room looks more traditional than the art, but these days people are using art to update various rooms in their houses. A piece that is 36 wide x 48 or so high could work well. I think that the vertical lines of a piece that is more narrow than high would help the space.