My head size was just below the jump from medium to large. I ordered a medium as indicated, and it was true to size, i.e. I was able to wear a medium - the fit is just shy of snug but not tight.
Some sites also say that MIPS makes a helmet less cool, but after 40 years of riding in Florida heat in many different helmets, I can't detect a reduction in cooling.
I still have no idea whether the MIPS feature will help in an accident unlessuntil I have one.
This Giro helmet is lightweight, streamlined, and comfortable to wear. It fits comfortably with an adjustable dial in the back of the helmet. The flat black color hides scratches and at the same time, is attractive. I would recommend this helmet to any bike rider who enjoy comfort and protection.