The first problem you have is that it is too cold there. (Definitely do not plant out in fall). You are likely to be looking at managing it as a container specimen, pruned to remain in scale and repotted periodically, overwintered with protection, like a bonsai. If it is a true 'Bloodgood' it will be handsome enough to merit this trouble, although there are of course dwarf varieties that would be more suitable for such use. Where hardy, 'Bloodgood' can grow over 30 ft. tall.

While Z4b is difficult, I wouldn't be totally pessimistic about Bloodgood's survival in the open garden for you. I planted one more than 20 years ago in a fairly exposed hillside location and it is now about 20 feet tall. It has experienced lows to -15 with no problems. Like many plants, hardiness will increase with age, so perhaps the best course would be to pot it, keep protected in a cool location over the winter and plant out next spring. The chief enemy of JMs is probably wind rather than air temperature, so protection in the form of burlap screen for the first few years may be a big help.

So if the tree is only 18-20 inches high now, what size pot should I use, and what type of soil please.

JM Bloodgood suvives where i am on the very edge of Z4B (unfortunately)