It probably has to do with the threat of them blowing over in a storm. I don't see why a longer trunk would require a significantly larger rooting area.

The larger tree roses produce a bigger canopy which requires more roots thus a larger container. Mine is planted in a tree pot which is larger than a half barrel.
I doubt they meant it "has" to be in a wine barrel. I'm sure any really large containers as big would work fine.

No I'm sure they meant the dimensions of a wine barrel. But I don't often see pots that large. Or maybe if I did I couldn't afford some of them. When you say a "tree pot" are you talking about a recycled pot that you purchased a tree in? As in plastic or rubber?

Karl, how do you move that big a pot? Currently the largest I have is about 24", and that isn't easy to move with a dolly.

At some point you do hit Clarence Rhodes idea of cutting down plastic garbage cans that come with their own set of wheels. I'm thinking those aren't much larger than 24" though.

I move heavy pots and a lot of other things with my heavy-duty moving dolly with extra big wheels that move easily over ground, lawn, and small bumps. If necessary, strapping will hold awkward sizes in place. It is surprising what a 110 pound woman can move with the right equipment. I have used extra large garbage pails as planters, it is ugly but it works.

I planted a tree rose in a huge pot a couple of summers ago. By the end of the summer the roots had actually grown out of the holes in the bottom of the pot & had started growing into the ground underneath.

I have 2 of the 36inch double delight tree rose.It can get so heavy that I have to get my husband to move it with me.one week ago I got this metal stand rolly so I can move them a little more freely also safe my deck.it cost like 15 dollars for the rolly each.happy gardening!!!