An adult size hat measures about 6 3/4”-7” across the circle before you stop increasing.
How do you figure out your size? If you have a tape measure go around the back of your head above your ears and then across your forehead. Mine measures about 23 in. To get the diameter, divide this 23” by 3.14. this comes to 7.32. That is when you are measuring across the circle and when we will stop increasing. So I’m going to make mine between 6 3/4” and 7” before I stop increasing. That will make it a little snug, that fits my head, and not too floppy.

Gauge= why bother?
I think hats are sometimes difficult for some people too make. Even if you use the exact yarn and the exact hook size we all crochet a little differently, our tension is different. If you would rather do a gauge swatch, here it is. If you would rather not, I will give measurements throughout.

8 rows x 12 HDC=4” 5.00 mm