OK, I just pulled JC's dress cover out of the closet. The label says it is a "7.5". Measuring, that sizing appears to be the diameter of the hat band. That makes (using this handy calculator) a circumference of about 23.5 inches.
That's more or less the same size as my head (with hair). I'm blocking this before (and possibly after) I attach the lining, and it's easily blocking out to 20 plus inches. It's remarkable how much the stranded knitting evens out with a good blocking.