I ordered a 'large' hat, which I thought would be more than big enough for me. It wasn't.
There is a 'Sizing info' link next to the size selection menu. Clicking on it opened a pop up, which gave a lot of information about sizes for trousers, shirts, etc., but nothing for hats. So I had to guess which size would fit me. I thought 'large' would be larger than I needed.
When I received the hat it was too small for my head. The label inside the hat indicated that it is for a 59cm circumference head.
I sent it back. It fitted in a post box slot, so no need to queue at the post office. But I had to pay postage.
It was replaced by an 'Extra large' hat, with a 60 cm dimension: a tight fit, liable to come off in windy weather, but it will do. The quality of the hat is fine.
One of the other reviewers "ordered the small and it says 56.5 cm on the label." My 'large' was 59cm. My 'extra large' was 60cm. No larger sizes are available. Other manufacturers sizes can go up to XXXL.
Wikipedia (though it says "the various 'standard sizes' vary from maker to maker and style to style") does have a table of hat sizes. Wikipedia's 'small' and 'extra large' are slightly smaller than 'small' and 'extra large' Highlander Premium Sun Hats.
Why, oh why aren't the actual dimensions of the various Highlander Premium Hat sizes in centimeters given?