Hi Chris, and welcome to GOY. The answer is - how long is a piece of string, in a way - because it depends on what tree you're hoping to plant in a tub. If it were an oak tree - you can't! Most trees really get too big to thrive in a restricted container....so what did you have in mind?

At the moment it just needs to be comfortably larger than the roots of the tree. As the tree grows you will need to repot into larger containers. Ultimately it will depend on what sort of tree it is.

Firstly let me also say welcome to GoY, Chris.
Lets start with the final height of the tree you want to grow in a tub. The diameter of the tub can be slightly smaller than the trees canopy spread except for columnar tree growth. The tub must form a stable base so it won't blow over. The tree must not be too big or you won't be able to handle it without damaging it when you root prune it every two to four years depending on the variety. You can start with a seedling tree (two years old) and a small tub transplanting into larger tubs as the tree grows until you reach the final tub size.
The tub must have plenty of dranage holes. The growing medium must be free draining, this means when you water the tree the water passes through the growing medium and FLOWS out of the bottom of the tub.
You can grow ANY type of tree in a tub (sorry to disagree, Spritzhenry). The only restriction is will the tree be hardy enough to survive the winter outside or will you have to take it indoors for the winter. All of our native trees are okay for tub growing all year.
I can't advise anymore as I'll need to know what type of tree you have in mind.
I hope this helps. If it doesn't, at least it gave you something to read. ;-))

I never mind learning, Stjohn...but I wouldn't want all that hassle, to be honest! I'd rather see (most) trees planted in the ground. Up to Chris, now. :-)

Well of course you can grow any type of tree in a tub - just as you can keep any type of puppy in a cage - to begin with. Eventually they both outgrow their containers. Ground Chris, unless it's a Bonsai.

Bonsai puppies - thats new! ;-))

LOL.