I have a question.. Why di dyou put it in such a large box? Now your tree has a lot of space to drown in.
I would have kept it in the original pot, and repotted in spring.
You can remove a reasonable amount. I try to cut back to clear side-roots, especially when cutting bigger, main, roots. It is a young plant and unless you now in one go fully remove roots, the tree should not drop branches due to root pruning.
You can cut back quite far too, but I would do this only when well established with the roots. So I would repot, wait till mid-spring and only once you get good growth, consider a light prunning. First styling can then be done next fall.