Well, if you want to do more than whisk at whimpy stuff, but actual cut tough brush, you'd best keep it 5 ft or under. That is already larger than your manual calls for; Implement page.
I would even say 4'. I have one that size on my B2620, and sure wouldn't want a larger one in the rough areas I use it in. Now if you are just keeping a reasonably level field in check, perhaps I would try a 5'. The nice thing is that 5' is the most commonly rented size around here so you could give one a try. Leave the loader on, or add some front weights if you do try a 5' bush hog though.
I have a 5' and it works fine. I used it earlier this year to mow an area that's hasn't been for a year. I'm sure it can be stalled though because the b2620 in hydro has only 19 Hp at the pto
I have a 2320 and i bough a 4' used cause it was a real deal but i was looking for a 5'. I never run out of hp with the 4' so i might think a 5' woud be the ideal size for the tractor cause i found my 4' very narrow for the tractor size.
I have a 5' on a 2920, the issue is not HP,,,,the issue is the size of the hog with all that weight cantilevered off the back,,you need the loader on, and ideally something in the bucket
I have a 4' Bush Hog Squealer on my B2620, and have brought the front end off the ground causing a touchy situation when trying it without the loader on. It was not a pleasant moment although it came out OK in the end, and I certainly wouldn't even try a 5' mower in any area that has steep spots after that experience. ( I haven't used my mower since then without the loader on either!)
I had a 5' landpride bush hog for my b2620, but it seemed a little big for the tractor. So I finally sold it. I just use the 60"mmm on the b or the zd326. If it is something that I let get overgrown, I just work at it slowly.