How do artists decide on the size of a piece? This is, assuming this decision is up to the artist, of course. Sometimes, it is not. I recall a call for entries from the California Watercolor Association. Only pieces about the size of a coffee table were considered. This effectively ruled out most field work, as it is extremely difficult to work on paper of that size outside of a studio. This in turn meant that most of what I had done could not be sent.
So if artists can choose size, how do they go about this? I can only talk about my own experience. Some of these choices are arbitrary and have to do with what's available at the moment, and how badly you want to hop in your car and drive to the art store. Some days, I'll just grab an odd-sized canvas or piece of paper because it's been lying around too long, then I'll challenge myself to find a composition that fits it.
More deliberate decisions usually have to do with time. A small representational piece almost always takes less time to complete than something double its size. This cabbage painting, for example, is 12 x 12 inches. It took me three two-hour sessions to paint it in acrylic, while a larger canvas would have taken me six or seven sessions. Had I decided to do it in oil, we would have had to add additional time, as some effects may have required the preliminary layers to dry a bit before I could continue.
Although some of us might like to think that the subject can dictate the final proportions, this belief has been challenged by scholars such as Anthea Callen. A chapter in her book on the impressionists explores the origins of old European standard sizes for marines, landscapes and portraits dating back to seventeeth century France. Callen found that loom size, the convenience of the merchants selling pre-assembled stretchers, and the artists' own priorities influenced size decisions more than the type of subject to be painted.
Sometimes I think of a piece's "presence" as I am considering the size I will assign to it. The same cabbage painting, in a larger size, would have been overwhelming. There is too much contrast and not enough subtlety for a larger size. Or what I paint may become too complicated for anything beyond a foot wide or long. There are days when I want to saturate the colors, and a small piece tolerates this effect a lot better than larger canvases, which would project a certain vulgarity.
Finally, there is practicality. I think of ease of transport and the expense of presentation. Almost all work done in the field is of reasonable dimensions (less than 24" long), given the challenges of that environment. Some colleagues paint in sizes they can store easily, while others choose works commanding awe in the gallery circuit, but expensive to move around. Large pieces are also more expensive to frame and in some ways more fragile.
No matter what the considerations are, size decisions are seldom made randomly, they follow artist's idiosyncrasies and practical matters much more more than any "rules."