Here's a basic hat 'recipe'
that works with the head size and your gauge to get the right number of sts to cast on. You usually make a hat an inch or two smaller than the head measures and heads vary. Some children's heads are as large as an adult's, my head is larger than my grown up son who's 6" taller than I am. This head size chart
may help you with an general idea of measurements.
For crochet I usually start with top down, then inc the circle until it's about the size I want (a bit smaller than the actual head) then work straight until the length is right.