I used the knitter's handy book of Patterns by Ann Budd to give me some direction for a hat size, but it was too big on the first try and so Henry's mother also gets a hat.
After finishing both hats, I think I know where I went wrong. I knit the sweater on a size 10. I don't have a short (or set of dps) in a size 10, so I bumped up to a 10.5 for the hat, but I cast on for the hat as if I was still using the 10.
Today's special reminder: GAUGE IS IMPORTANT.