This is not the first Irish tweed cap from John Haney that I have purchased - they are all top quality, well-made and more important - they fit!
I get the men's small (listed as a man's hat but I like to keep my head warm too); I do measure circumference of my head before ordering, but so far, the size small fits me well without falling off (another brand I had to return as it was too large, even though marked 'small').