Not that I have a lot of experience or anything, but I recently bought a 60" tiller to use with my 26 hp tractor. You have the lift capacity for a 60" and probably enough HP to run it in previously plowed or broken soil or sandy soil, but hard packed will probably be a little strain on the tractor but that can be offset by lighter cut or going a bit slower.
A 48" will definitely cover your tracks which you definitely want.
I think maybe the big deciding factor on size is how you plan to use it and what type of soil you use it in. Caroni and a few others offer a 54" or 56" that is probably the perfect size for our tractors.
I planned on buying a Caroni 56" but it cost more than the 60" at agri supply & after seeing them and comparing a few different ones I liked the KK copy they offered better, and after using it I'm glad I did. My soil here at home is sand and I can't even tell the tiller is running in it and over on other land we have it is a heavy clay and I just had to go a little slower and take a lighter pass at first but the second time around I was able to go full depth at a decent pace and it did a great job. Was able to make 2 passes on 2800 sqft area in about 40 mins, a lot less time than I thought it would take.
Good luck .