Here is the article on fit from Walz:
The WALZ “Perfect Fit” Guide
How to Measure:
From pinheads to melon heads or those in between, we've got a cycling cap that will fit perfectly. Unlike other cycling cap manufacturers we offer two sizes: small/medium and large/extra large.
How to Measure: Take a piece of string or flexible tape and measure directly across the forehead, above the ears, and around your entire skull. If the measurement is 21.25 in (54 cm) to 23.5 (60 cm), order our size 'small/medium.' If the size is greater than 23.5 (60 cm) and less than 25 in (63.5 cm), order our size 'large/x-large.' Simple.
How to order
In the section below, fill out a row for each style you want. If you want more than three styles please fill out a second form. Be sure to enter a quantity, size and color options you want.
Cap price is based on reaching 50 caps for the entire SFR order. Next price break is at 150 caps.
Price includes shipping to each orderer.