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.
I have a 5' Bush Hog Squeeler on a B2920. As others have said, it is a lot of weight setting way back. If I was buying today, I would probably look at the Woods 54". I tend to drive with my wheel on the edge of where I just cut on the previous round anyway, so I would really only be loosing 3" on the inside side of the cut. Meanwhile it is smaller and lighter, but would still cover the tracks of the tractor.
Another thought, if you are mowing reasonably thin grass with no rocks, logs, etc in there and not wanting to clear brush, have you considered getting a rear finish mower instead. They can be set to mow pretty high. The weight is not carried as far back as with a rotary mower. Also, their wheels support the mower when it is on the ground.
One of the weaknesses of using the Kubota tractors with the 1/4" valve is when mowing with a rotary mower. The 1/4" valve will not hold the implement at a set height for a prolonged period. Mine will drop about 3/4" in 15 minutes, so while you are mowing with a rotary mower which relies on the 3 pt hitch to hold the front of the mower up, will sag down and if you forget to bump it back up, it will be dragging on the ground in the front. Mowing with a RFM would be much easier if the conditions of your pasture would allow it.