I'm not an expert on hunting, and if a real expert wants to submit a FAQ here I will gladly defer to it. Until then, my humble best follows:
If one peruses ammunition catalogs, one will find that all .223Rem soft-pointed factory loads are rated as suitable for hunting "varmints, predators, and small game". This means that a .223 rifle can be used to hunt anything from the size of a medium dog (40 lbs) on downwards.
A .223 rifle is definitely not suitable for hunting deer. It does not have the bullet size or power needed to make a clean kill, except in unrealisticly favorable circumstances. Hunting anything bigger than a deer with a .223 is out of the question.