As large as you can manage! A half barrel would be good. Place the container where it will live as it will be heavy when full. Use John Innes no 3. Keep it well pruned or it will lose too much moisture. Feed regularly, too.

Thanks volunteer. I have seen a 40cm pot that i like. Will that be suitable?

Can you also recommend an organic compost please?

Well, J I compost is a soil based compost but it does contain a little peat. If you google John Innes compost you can see the ingredients and maybe mix your own using something else for the peat part. The whole point is that it is a soil type of compost (hence it being heavy) and is better than the peat alternative types which are not so suitable for growing a tree/shrub long term in a pot as they eventually decompose. Soil based composts don't decompose. Hope this makes it clearer!