How big pot is useful?

Hi, I have trees that badly need to grow in oversized pots. Or in the ground. But I keep thinking how large should the boxes be?
One of my birches have a nice rootspread and I would like it to be planted in a shallow pot. But it needs many years before it should get a suitable pot.
I cut it back hard earlier since the branches was no good. But it gets me thinking. It was completely potbound. One hard mass of roots, so I put some new soil around it so the water didnt run just straight trough. Considering making a very big "round" wooden box for it, but how much larger should it be?
The pot its in now is about 8x6 inches or something. Ive heard some contradicting opinions on this, and see several dig up trees from nature and crank them into just a bit larger pots, while others say the box should be alot bigger. What do you think?

Sincerely
Storm

We don't do many decidious trees in Florida such as Birch, so I can't speak directly to your question and I am not really sure what you are asking.

Generally speaking you don't want a pot with too much free soil because that soil, without roots, will stay too wet and encourage root rot. I thnk you are saying that you don't plan to disturb the existing soil, I think that is a mistake, I think you schould consider two or three pie cuts and fill them with new soil.

What I would suggest, bearing in mind the above comment about Florida.

Find a pot two inches larger 6 x 8 go to 8 x 10, but also make two pie cuts now and fill them with new soil. In a few months, if the tree looks healthy, take one or two more pie cuts.

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For a tree in development, my rule is that the new pot should be just big enough to get the tree slightly root-bound at the next rempoting, which I do every 2-3 years. If the tree is not chopped during this period, a little less than double the volume of the former pot is ok.

Agree completely with Billy and Alain. In the ground is a different matter though. Fastest development comes with unrestricted root growth (perhaps with a tile or similar to prevent downward, tap root growth).

Thanks for your answers.