Marion, it's hard to say what size pot will be adequate but how about if instead of a pot use burlap? Or something similar..Then wrap the burlap with brown paper so soil doesn't leak. They used to sell pot sleeves for relocating plants in pots..sleeves slipped up pot. Sizes vary, depending on pot size..the last time I found a site that sold sleeves, they came in 'x' quantity, in other words you couldnt buy one..but, if you know of a nursery that uses them, they'd probably be willing to part w/a bag or two..I'd get the largest size, then insert tree inside, and staple excess paper.
Also, digging, then moving is going to stress the tree quite a bit..roots probably will break so be extremely careful when digging..
Your tree will probably lose leaves but w/care should return. Good luck, Toni

Thanks Toni. I have a friend in the nursery business I'll ask her about the sleeves.