When you import the image, it will resize your window to fit the image, so whatever the largest you want your document window to be should be the size of your image.
Try to get the best quality image for the smallest size you can.  I know that RDNA has a lot of good background images in their freestuff.  They're nice, they're small, and they've got excellent lighting and detail :)  If nothing else, I'd use them as a comparison on what works really well as a background photo.

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Knowledge is knowing a tomato is a fruit. Wisdom is not putting it into a fruit salad.