Younger stock needs comparasons. How others have pruned their privet aught to color your thoughts on how you might want to do it.
This is one of the few trees in your new collection that might get top pruned. Look over the other guys shoulder some. And then sit down next you your tree with a pencil and paper. Draw twice, and cut once.
Sym, your finished size will be up to you. I would rather urge you on the side of caution; please leave on a terminal leaf (at least) on each branch you prune back. Privet might bud back on a branch without a living terminal leaf. But then again it might not...