I foot of rotary cutter width per 5 pto HP is the rule of thumb. For example, a tractor with 25 pto HP can run a 5 foot cutter. Pto HP, not engine HP. Pto HP is always less than engine HP.
With your 13 PTO HP - a 48" (120CM) might be a the right size anything larger will be too much - really depends on WHAT you are cutting and how thick and fast you cut it.