I wouldn't go smaller than 16"-18" wide. Maybe there's a "true" formula out there, I don't know, but like you I think art should "breathe."
Have you tried this trick? Using old wrapping paper or brown craft paper or a piece of posterboard from the $1 store, cut out the size of art you have. Tape it up; live with it for a couple of days. Then cut a smaller piece from another piece of posterboard, a few inches smaller all around, and tape that up instead. You'll see which really looks better to you. Really, it's all about what looks right to YOUR eye. Your husband may see the difference between the two or not. (Did he say, "Looks fine to me, honey! Let's watch TV"? LOL!)