you need to wear it when you feel like your too cold... personally, i never wear a beanie under my helmet, im warm enough with my helmet (not my face) and that goes to like 15 degrees or so.
mine are kind of medium sized. but with my old helmet i had to wear super thin ones because my helmet was too small. do you wear your goggle under your helmet?
um under with a frames because i heard the strap snaps over the helmet and over when i get my custom crowbars. i bought a medium just fyi. probably doesn't help but i figure i can add more info