As 3D artists, the size of the render we submit is totally up to us. All we have to do is to be willing to "put up" with tying our computers up while the image renders. This can take anywhere from mere minutes (the Marlin only took 15 mins) to hours (like this room!) It seems like the more complicated the image is, the longer it takes to render.
I have renders in my own portfolio that have taken as long as 48 hours to render. Others that took 12 - 18 hours. I've never minded until now because I was willing to leave my computer work that long.
But when does it stop being about the max size and and start being about time? Simple isolations that are not meant to be realistic take the least amount of time to set up and render, no matter what software you are using. The render I mentioned that took 48 hrs looks very realistic, but it only has a few downloads. I had rendered it to be 6.8MP. When I look at the size of the images downloaded compared to the time it took to render it this large, it was such a waste of time! I had been rendering things as large as I could so I could offer all the possible sizes, but now I'm not sure that's wise. I have a number of files started on my computer that I haven't finished because it would take entirely too long to render them at 4000 x 3000. When I look at the sizes that are being downloaded from my portfolio, I could easily get away with simply rendering them at a smaller size.
If you also have files that are gathering dust because of the render time, take another look at the size you intend to render them. Sometimes bigger isn't necessarily better!