A large one is the short answer.
However, you need to peruse your manual re. your tractor's 3pt lift capacity, and the pto requirements of the particular brush cutter. Folks on this forum tend (unlike the previous poster, who listed weight) to lump similar width brush cutters together. There are major differences between brands, and within brands the level of "duty" rating. For instance, within the Woods line (which, admittedly, is an expensive product line) at 84", the "standard duty" requires 35 pto hp and weighs 830#, the "medium duty" requires 40 pto hp and weighs 1280#, and the "heavy duty" requires 65 pto hp and weighs 1570#.
The other issue is what do you wish to cut?
If you only need to cut tall grass and weeds, most "standard duty" cutters are fine (and many of the lower line brands have only one model per width - and it's "standard duty" without being so stated). For most other stuff, a "medium duty" is fine - heavy brush, 2-3 inch saplings, etc. I've only seen a "heavy duty" Woods cutter on a large tractor used for forest work - it was a monster.