Simple as can be; measure, in cm, around the head. just about 1 to 1.5 cm above the ears and eyebrows. For a small head this will be about 54 to 56 cm, for a medium head about 56 to 57 cm and for a large nut about 58 to 60 cm
Here is a conversion chart to go back to the old hat sizes
https://www.hatsandcaps.co.uk/pages/hat-size-guide