To find out your hat size, you need to measure your head with a tape: wrap the widest part of your head just above the ears. For the accurate result, do it several times. Do not overstress the measuring tape. If the measurement is 57 sm, then your size is 57. If you get not a round figure, it needs to be rounded up. The most common sizes for adults are 56, 57, 58 and 59.
If you happened to buy a hat that is a little large for you, try putting a small layer of foam rubber or any other soft material under the inner belt of the hat. If the hat is too small, it would be harder to fix. Refer to your hat master.