Well I've been researching this a lot since many people told me my 1-gallon pots were too small....

But every growing bible I've read said 1-4 gallon is acceptable.

From what I've read a 24" plant will only have a root-ball the size of a baseball. Once they reach 4-feet high the root-ball can reach softball size. It is that point where they become too large for a 1-gallon pot.

If you look around at other similar plants and trees the root are of similar size and don't really need a large area to mature.

However as was said the bigger the root the bigger the plant.