Ok, well, I wear balaklavas with helmets. I have about 8 balaklava's ... some are "thick" and some are "thin" ...
One of my helmets is a size larger (1 inch) than fits tight. That one is for when I wear the "thick" balaklava's .. then other helmets fit tight but work with the "thin" balaklavas. Last winter, for instance, we got down to -20F a few days ... with 2 weeks of totally under 20F. So during that period, the thick balaklava's were in use with the oversized helmet.
So, depends on your environment. So might need two sizes of helmet if you would wear the thick ones. If just the thin, you'll be fine with one helmet.
And you do need to measure your head when you order the helmet !