Length is 1.85 and a .223 is 1.760. I know you can make regular .222 out of .223 cases (which is a pain in the ass) but I'm not sure you have enough metal to stretch for the magnum.
Do NOT try and fireform a case that is too short for the chamber. The end result is more than likely going to be a split case and the front half stuck in the chamber.
I suggest trying .204 cases. 222 magnum is it's parent case if I remember right and the same length. It should be just one pass thru a sizing die and annealing