How much of a subject should be included in the image? That’s always a compositional question: exactly where should you position the edge of the frame? When shooting with a fixed focal length lens, you really don’t have a choice unless you can physically change your location. If you’re using a zoom lens, you have an almost unlimited choice. Of course, no matter the lens you could always crop the image afterwards, but all cropping is lossy and personally I hate to throw away data.
So just how much, or how little, needs to be included? Consider these two images. Your thoughts? Is the roller too small in the frame, the elephant too tight? What do you think?