All our sizes have the same construction and overall shape. The circumference (B in the image below) of a cap is adjustable within 1-1,5cm and maximum circumference increases from one size to the next size. Secondly the depth of the cap (A in image) increases from one size to the next size up. A Medium cap has therefore a greater depth than a Small cap and a shallower depth than a Large cap.
So let's say your head measures 58,5cm in circumference (Measured accurately where you want to wear your cap). Your head will fit in both a Medium and a Large, but a large will with its greater depth (A) come lower on your head and rest closer to your ears.
So in order to choose between the two, try to reflect over other caps you have used and how they fit your head. Do caps normally touch your ears or come close to them or do caps often feel shallow and not deep enough when you try them?
Look at our videos on how to measure your head and understand the depth of fit below.