Most of my ready made hats have a head size opening of 22 1/2 inches, which is average. If your head is a little smaller, depending on the style of the hat, this should fit you just fine. You may also improve the fit by inserting a bit of padding beneath the lining. Most of my hats have a drawstring fabric lining that may be opened in order to accommodate this. If your head is a bit larger, you may still be OK—-the hat will just sit a little higher on your head, and if it has ribbon ties, should stay put. You may also employ a hat pin to help it stay in place.
If your measurement is more than about 23” you may wish to commission a custom-sized hat. (link to commissions)
Head size is typically measured with a cloth measuring tape around the head at the widest part—-usually around the forehead to the back of the head, holding the tape parallel to the floor. Period hats often sit well back or high on the head, so, alternatively, you might measure your head around the line where you want the hat to sit.
Consider how you will be wearing your hair—-if you will be wearing a wig of hairpieces, or if you typically wear a cloth cap under your bonnet, you will want to measure accordingly.