I second what Kytim said. I have a 48" for my B7100, and it pulls it good in my soil.
The 7510 has 5 more horsepower than the 7100, so unless you have some abnormally dense, hard soil, your tractor should handle a 48" fine. If it were mine, I wouldn't go larger, though. Rule of thumb is 5 horsepower per foot, so 4 foot put you in the ballpark.