I have a 6 foot Woods Box Blade on my NH TC33D with loaded R4 tires. I find it a good match for the tractor. Your JD 4300 is similar in capability to the TC33D, so I would figure you could handle a 6 footer.
What kind of tires are you running? are they filled or weighted? What kind of work are you planning on doing with it? For maintaining a gravel road, a 6 footer should work fine. for serious earth moving in difficult soil conditions, it may be too much, but then you'd probably want a dozer for that anyway.
John Mc