Assuming you are seeking a 3-point rotary cutter, your machine would be able to spin a 6' (72" is the standard descriptive term) but it would perform better with the 5' (60") cutter.
The weight of the implement is a serious factor to consider; any 72" rear cutter is going to 'feel' heavy for your machine (imo) but there's probably members here who will tell you to go for it.
If your'e going to acquire one, I'd recommend the 60" in a known brand so that replacement parts and/or service is readily available. Some of those brands would include Land Pride, Woods, and perhaps a few others. I've never heard of Southern Attachments, but whatever you choose good luck and show us when you get it!