I'm currently working on a hat. This is only the second hat I've ever made, so I'm still not 100% sure of myself. The first hat I knit on 24" circs because my head measured 23.5 inches, so I figured it was close enough.
When I went to the yarn store this weekend to pick something up, I was talking to one of the owners, and she told me never to use anything bigger than a 16" circ. She said even if the circumference of your head is close to 24 inches, you're still stretching the yarn. I have asked her for tips/advice before, and I do trust her.
However, I'm knitting this hat (and I *did* take the time to swatch and measure my gauge, etc.) on 16", and compared to my first one, it just looks tiny. But I measured, figured out how many stitches to CO, etc. I should just trust the math, right?!? 16" is the correct size needle?