Hat sizes can be tricky. Especially when you are making one that you are doing free hand or the pattern may not state how long to make it. There is nothing like making a hat for a child that is too long or for an adult that is too short. While working on the Zombie Hats I ran into this issue a couple times. I found a chart and took a screen shot that seems to be a wonderful reference. Unfortunately I can not remember the video I took it from. *sigh* So if I do run across it again I will post the video.
Now this is for the measurement from the top of the hat to the bottom. Seems to work out as a wonderful reference.
Happy Crafting!