Wider blades cover more ground & therefore "average" better- in other words. you will spend less time leveling an area than with a narrower blade.
However, narrow blades dig better; less surface area to "drive" into the ground.
My neighbor has a 20hp B2150 with a 60" box blade and his grades pretty well, if there isn't a lot of digging to do.
My 16hp B7100 with a 42" blade (same width as the tractor) will dig circles around his. Our ground is very hard, and if there is digging to do, (such as cutting a water course for drainage or a dich) he calls me. My blade is as heavy or heavier than his wider blade, so it just cuts better. But mine does not level a large area as easily as his 60"; it will do it, but it takes more time, passes, & effort.