I have only a little experience with a box blade, but here is my two cents. You definitely want something that covers your tire tracks. I have a 6' blade on my tractor, which is about 53" wide. Not only do I cover my tracks, but the extra few inches that it sticks out means that I can get close up to a structure or fence if I need to, without having to sidle the tractor itself right up to it. IMO, if the box is a bit too big, you can always adjust the top link for a less aggressive cut or lift the box slightly to dump some of the dirt.
To be fair, the counter-argument here is that covering your tire tracks is less of a big deal with a box blade than with something like e.g. a landscape rake or a land plane, where you're doing more "finish" work, or a tiller, where the light, fluffy surface is what you're trying to produce and the tires simply ruin it. And maybe you don't need to get that last six inches close to the building or the fence-line. Also, having an implement that is narrower than your tractor means you don't have to look behind you every time you drive through a gate to make sure you're not going to snag the implement on the gate-post. And you may get work done more efficiently and quickly with a small box that your tractor can muscle around, compared to a big box that you have to use kid gloves with.