I use a 5' on my 2520. I guess it depends on your soil and what you plan to do with it. We have mostly sandy soil,and it does well here...it will strain for traction as the box gets full though.
I went with 4' on my 2320. My tractor has no problem with the 4' BB so far. I debated the 4' or 5' but finally decided to go with the 4' because it covers the tractor's width and shouldn't be too much for the tractor where a 5' might be.
I've been doing alot of info searching myself,concerning BB size for my tractor.
If the 5' is pretty much max for a 2520, the 4' would be best for the 2320.
my.02
Well I barrowed a friends 5 foot King Kutter box blade to see how the JD 2320 would handle it. I smoothed out the paddock area around the horse barn. Then did some maintenance on our motocross track. The 5 foot BB worked like a charm behind the JD 2320. So I traveled over to the dealer and picked up a new 2009 left over 5 foot Woods box blade for a good deal. The 5 foot BB was $380 less than a 4 footer. I do not plan on lowering the scarifies all the down so I believe the 5 foot BB will work just fine with out putting to much stress on the tractor.