Men's shirts are always sold according to collar size in Britain. In the case of casual tops and T shirts, they might be S, M, L - small, medium and large - with 'extra small' XS and 'extra large' XL - or even XXL - as additional sizes at either end of the scale. Size + a number is a woman's dress size. A (largish) woman would say 'I'm a size 16', but a man wouldn't say this. He might say 'I take a sixteen inch (collar)'. Sixteen inches is 41 cm, which is an average/medium size.

Yes, in Britain, men's shirts are always sold according to collar size. The 16 in your example refers to inches. "I take a 16" inch collar." "I am a 14 and a half." Collar size gradation is by the half inch. Shirts made in Italy are sold by the collar size as well, but measured in cm. A 16" collar shirt is known as a size 41 shirt.