I would look for a 36x36 square size.

Make a mock up of different sizes in newspaper, and tape it up to see what you think.

Here's a link with some helpful info: http://www.drivenbydecor.com/2012/08/20-rule-of-thumb-measurements-for.html

It should hang about 9 inches above the top of the buffet. I agree with avery2pto on the sizes.

If you look up dining photos with buffets, there are a lot of ways you could go. Depends on if you have lamps there, or other things. You can do one large painting or mirror, or a couple of prints, or even layer art pieces:

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I would paint that wall behind the buffet the same color as above the chair rail.

One large art piece is also wonderful:

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I would go big and as for height as close to the buffet as possible maybe 2 or 3 "

Wow, really disagree with some of the advice here. People tend to pick art that is too small -- I think something scaled as for a couch is appropriate. One of these choices, say:

Good place for a large painting, poster, or something like that -- can always get a placeholder until you fall in love with something...

I think it is hard to go too large. You want impact. If you are really unsure, before you invest in anything very expensive, go to World Market and bring home a few in different sizes. Returning things there is easy.

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If you have an old calendar, and the pages are usually 12in by 12in, or 12in by 11in, then tape the pieces together to create a 36in by 48in and then other sizes to get a feel for the size that works for you.

I agree with larger artwork as well. At least 48" wide, and you can also go tall as @pcmom1 is showing above. Don't be afraid of large artwork. It'll work great in your space!

https://www.etsy.com/shop/ShoaGallery

One large piece; it's the better balance to the panes of the hutch and cabinet doors of the buffet. You have a lovely simplicity to the room; one large piece of art would complement it nicely. Roughly 48" should do the job. You don't have to worry about filling the height so much; high ceilings are beautiful, but treat them as normal when hanging art. Keeps the picture at a comfortable viewing height, creates coziness and lets the ceilings have their own shine.

As the other pros have said, one large artwork. With your sideboard being 72" wide a rectangular piece 60" W would have great impact. To balance the height of your china hutch, a 48" square would be great. We have a wide range of canvases available in these sizes (100 x 150cm or 120c x 120cm). Feel free to take a look at our online gallery to see if we have something that resonates with you.