I have a four footer for my 790. The reason for a four footer is the use of it in the tree farm, where width issues come into play when in use in-between trees. Its a frontier BB2048 and a heavy, stout little thing it is.
So, working area should have some say with the size of the implement, only time I wish I had a wider box blade was leveling a larger area of about half acre or so.
For removing brush and small trees, I do not believe a wider box will be of advantage because of finding large and or many roots with even one ripper, its a hard pull, and just gets harder with more rippers down in my experience. But for driveway work and yard leveling, a wider box blade might be the way to go.
I would get a heavy duty box blade regardless of the width you choose also.