Official recommendation on my Kubota 22hp is 42-in rotary cutter (if I'm remembering right). I just yesterday bought a KK 4-ft rotary cutter from Tractor Supply for $571. It was the best price I could find locally and couldn't justify dickering with them. It's not on their website, you have to call your local store. All my local stores had one in stock.
I would tend to agree with others, your tractor might be able to handle the 5-ft but if you're going to be cutting heavy brush go smaller. I feel I'm as big as I can go with my tractor.
Happy Implement Hunting!!