Here is a basic guideline for the hats that I make. I always measure my crown before continuing onto the non-increasing rows. You might have to play around with hook size depending on how tightly you crochet. I use worsted weight yarn for the monkey hats and a chunky yarn (size 5) for the owl hats, but use the same scale. If anyone has any other suggestions of how to measure finished hat sizes before finishing and finding out the hat is too big or small, let me know and I'll add the info in! Hope this helps!
*When I say "crown size" I mean the size of the circle before continuing with the non-increasing rows measured from one side to the other. When I say "hat height" it is from the centre of the first row to the bottom of the hat.
It should also be noted that this is a use at your own risk chart, as it makes sense in my little pea-brain, but might not be accurate for everyone.
AGE HEAD CIRC. HAT HEIGHT CROWN SIZE # of increasing rows on
0-3 month 13-14" 5.5"-6" approx 10.5 cm 4 rows G hook
3-6 month 14-17" 6.5-7" approx 11-13 cm 4 rows H hook
6-12 month 16-19" 7.5" approx 13-15 cm 4 rows I hook
12mo-3 year 18-20" 8" approx 14-15.6 cm 5 rows H hook
3-10 year 19-20 1/2" 8.5" approx 15-16 cm 5 rows I hook
Alternately you can try experimenting with the math equation C=πD where d stands for diameter, and c for circumference. For example if you want the circumference of the hat to fit a 41 cm head you want your diameter of the crown to be approx. 13 cm because (π)(13) would equal a 40.82 circumference. But thats just the math geek in me talking :) You tube has a great little tutorial for the mathematically challenged.