Retracted- I found answer in search archives
What was the answer?
Seems like 3 gallon suited quite a few people.
I just grafted mine on M111. I use rootmaker rootbuilder II containers. I made my container size decision based on the size of the initial root ball on my M111. I put a couple in 3 gal RZB2s but I put most in 1 gal RB2s. If the 1 gal RB2s become difficult to water during the summer, that means the roots are filling the container and I’ll transplant to 3 gals. Unless you are real far south, I doubt you can outgrow a 3 gal RB2 in a single season.
Some of my fastest growing trees are chestnuts. I start them from nuts around Christmas indoors under lights. I start with Express tray 18s for the first 12-16 weeks and then transplant to 1 gal RB2s. Late spring or early summer I transplant to a 3 gal RB2. I’m in zone 7a. Last year I planted them in the field in early October. I had chestnuts that were over 6’ tall and 3/4" in caliper. While they were ready to transplant from the 3 gal RB2s when I planted them, it was on the early side.
Depending on how far north you are, you may even get away with 1 gal RB2s all year. Also, keep in mind that most root branching occurs in the last 4" upstream from the prune. So, you get a denser root system by using multiple containers. If you directly with a 5 gal RB2, I would suspect there would be a lot of unused space in the middle of the container.