60" without doubt. My 2320 will pull a 60" and it has less horsepower than does the 2520 and a little less weight, so I doubt a 2520 would struggle too much doing so. I personally went with an aftermarket 54" for my machine (2320) to better match what the machine could easily pull with a box blade full of material, uphill on my property. I have owned larger JD tractors, including several 3000 series and two 4000 series machines and I can say a properly-sized box blade is faster and more efficient to use than getting one that is too large and cumbersome or that the machine cannot effectively pull. My 3720 would pull a 72" BB left over from my previous 4520, even though it weighed about 275# more, in addition to being wider and deeper. It just would not pull it well. Even ballasted, a 72" box blade on anything but the most level ground or the lightest density soil is too much for that sized (3000) tractor. People do it and some do it well, but not where I live. Consequently, the larger 2000 series can handle 60" or even 65" box blades, but getting much beyond that in size defeats the purpose as the machine cannot handle the weight and material that well and efficiency suffers. Interestingly, I have found on my property with box blade work, especially before our driveway was paved that the difference in actual time to pull and smooth road bond or gravel or dirt was not that much different, even between a 54" and a 60 or 72", so I cannot see any real benefit to going oversized, but one definitely wants to cover the width of the rear tire track on the machine. As for manufacturer, Frontier is my personal choice due to many of them being made for JD by Woods and Befco. Those companies also offer excellent box blades under their own name and usually offer more available features and sizes, too.
John M