I would go with a pot a bit larger than the current
root ball. Tuck him in for a year or two and revisit
Grafting is the process of transferring plant material
from a select form/variety and through various tech-
niques making it grow on the root stock of another plant.
Most fruit trees are produced this way; nearly all roses
are grafted and many ornamental plants/shrubs are
as well. It lets you produce hundreds of plants with the
assurance that they are all the same "type/variety" you
want. Many things just will not root with sufficient
success to make their production profitable in any way
other than grafting. BUT only the part above the graft
will be the variety you want--the lower part is usually
A lot of maple (Japanese cultivars especially) are grown on sturdier stock. A branch from the desired tree is clipped off and grown on that sturdier stock's root/trunk.
When you see a marked difference in the bark or diameter of a trunk you should suspect that the tree was grafted. It's not always a bad thing, but a poorly done graft will affect the tree's looks forever.
Your tree seems to have some signs that it was grafted where I marked your pic.
Do some more reading here and you'll see that you need to reduce the root ball and the total height, though that is a matter of taste, not just because of the graft.
And it also looks like the growth of the tree is emanating from the stock part below the graft. Maybe the top is dead even? Can't tell by that top branch on the right.
Yeah I think I'll get one of them huge garden pots for it and cut that long straight bit of when it's healthy so it tapers nicely just hope it will be OK till Friday that's the earliest I can get the pot