Medium. They’re a bit of a chunky looking helmet at the best of times.
I love my POC helmets. They’re expensive no doubt.
You can try pushing on the helmet from different directions with an open palm to roughly see what will happen while hitting the deck. The shell should shift a bit IME.
IME it will twist under your temple area and squish once the weight of your body drives your shoulder and head into the ground. As long as the harness/straps stop it rolling off you’ll get the protection you paid for during the initial impact.
This goes for any bicycle helmet.
POC from my childhood. So aero.
Nobody wore them.