I have a 4' back blade for my little B7100 that has worked well for me over the years. That said, if I had to do it over again, I would get a 5' blade. The reason is that when the blade is angled, the blade only barely extends beyond the tires. Also, I have to hang suitcase weights on the blade to get it to cut into dirt very well because it is so light.
If your tractor will lift a 5' blade, I would recommend that.
Vassar Equipment makes very solid implements for Cat 0, 1, and II lifts. The attached pic is of their "Estate II" rear blade for Cat 0 and Cat I lifts. You can get this blade in 4', 5', or 6' widths. They will sell to you directly. The shipping costs might be the only downside as they are in Oklahoma.
Click on the "On-line Catalog" link on their home page: Vassar Website