Newborn - approx 14" head circumference, approx 6" hat height
3-month - approx 15" head circumference, approx 6.5" hat height
6-month - approx 17" head circumference, approx 7" hat height
9-12 month - approx 19" head circumference, approx 7.5" hat height
Toddler - approx 21-22" head circumference, approx 8" hat height
I make these patterns in my spare time, so if I don't have the size you are looking for, hang around for awhile and I might get to it.
If you'd like to adjust it yourself, you should add increase rows, with less increases each time (For hats in double crochet the pattern of rows would go as follows:
2dc in each,
2dc dc dc;
2dc dc dc dc, etc)
until the circumference is slightly less than the right measure, then at the end of the pattern add length as well.
This is a good resource for the approx measures of different hat sizes: