basic beanie type hats basically are a tube with a flat top. They get shaped when you wear them a bit. Here's what you do to size them:
(your mom told
you used use math
1.measure around your head with a measuring tape for most adults this should be some where around 20 -23 inches
2. Divide that number by pi, 3.14
3. What you should have now is a number somewhere around 6 - 8.
4. Crochet a flat circle whose diameter (the measurement across all the way across the circle) is the number that you came up with in step 3. If you are use a really stretchy stitch you may want to make it a little smaller. Or if you just like a tight fitting hat. This is the part where you can do some fiddling with the fit.
5. Once you are happy with the diameter crochet evenly, no increases or decrease, until you like how long your hat is.
Hope that helps