In my own opinion, I'm thinking about the theory of hops and they're surface contact with the beer. Too small of a bag, screen, tube, ball.... etc: you limit the surface are of the hops and it's contact with beer. Too large of a bag, screen, tube, ball..... etc: well, can't think of a reason why too much surface area is a bad thing especially when it's going to be filtered.
I use the 18 inch tube and I put up to 3 oz of pellets in there, but I haven't tried more. I only use 3 oz max to dry hop a 5 gal keg. It looks like MAYBE another oz will fit comfortably with still enough room for surface contact.