The Owner's manual for my Kubota lists items like that and other attachments. Granted it really lists the maximum size and heaviest items not necessarily the optimal ones....
A cutter that it as least as wide as your tractor would be preferred -- a 6' cutter would fit that bill. The major manufacturers offer light-, medium-, and heavy-duty cutter models in various widths, and provide recommended tractor PTO HP ratings for those models. For example, Woods offers a light-duty 6' cutter that requires a minimum of 20 PTO HP (Rotary Cutters). Woods recommends at least 30 HP for its heavier-duty 6' models and all of its 7' models.
A light-duty 6' model should do fine in grass and small brush, but will suffer in larger brush.