My solution is that I knit it once, and try it on. If it fits, woo hoo. If not, I gift it to someone bigger or smaller than I am, and knit the same pattern again, with adjustments. Most patterns turn out better for me the 2nd time I do them - I can always see things I wish I had done differently. Hats are small enough you wouldn't mind doing a double. This isn't as good a system as many I'm reading here, but it works for me.