i think what ive done in the past is follow directions for measuring your head for a hat, and then use that number for a sizing chart to get glasses. the US uses inches down to the 1/8th of an inch.

it’s measured like forehead around the peak width of the back of the head, i want to say. snug but not tight.

i have a giant fucking dome so i have to special order shit if i want it to be comfortable as my dome size is usually XXL or 3XL on any charts or descriptions.

The 3D printing community has a number of designs.

Zenni app is p accurate with measuring