All sizing for helmets is referring to head circumference measured around the head just above the eyebrows. I try to leave a little extra room when printing for foam padding to be added. This is to make it a tight and comfortable fit when you are planning to wear it. Because of this if you are a half size I would suggest going to whatever size is closest.
Sizing for armor sets and armor pieces are based on shirt sizing, however if you are planning to order a full armor set please message me first so that I can give you a list of the measurements I need to make a digital model of you to fit the armor pieces onto before printing.