I cut my Privet well back in the Autumn. I have a small Box in a pot, just give it a trim at the same time.
Both will sprout shoots again next Spring.
They are pretty tough.

I suppose now if about OK to cut it back - any new growth should be mature enough to not die back in winter.

5ft wide to say 2ft would mean cutting into old wood or at least parts of the Buxus that has little/no light. If it regrows new shoots (if) it will take a few years to show any meaningful growth.