Over the last 23 years, I believe I have sold more hats one at a time than anybody else on the planet ( having traveled to trade shows around the world selling Walkabout hats one at time to some amazing customers, many who have become great friends )
Its a fact that not only do Hats change their size over time ( due to heat, humidity, use or more over non use, sitting in the cupboard, moisture etc ), our hat size can change, while our skull size cant change our hat size can change all be it slight, due to weight change, as well as hair loss or thinning. First you need to measure you head. Take a soft cloth tape measure or string, and wrap it around you head ( best if somebody else does this for you ) about one inch above the ear. Pull tight until UN-comfortable and then relax the tension. Do it three times you will get three different readings, take the average that's your size.
When picking out your hat, any hat be it a $2.00 cheap straw or a Walkabout hat even that special Stetson $1,000 Felt you always wanted. Be sure to buy it slightly loose, if its feels a bit to big, that's perfect. The more you wear it the better it will form fit to your head shape, the key is shape not size. Its a fact the more you use and wear your hat the better it will fit, its a function of moisture.
Many folk wet there hats on purpose, some downright soak them in the river while fishing ( my favorite method as it forces one to take a holiday ). Be sure to dry it on your head. Over time with use it will fit like a glove, dont worry you cant hurt it, in any case a little character makes the hat even more special.
If the hat ( ye ole 20 year old Walkabout Hat ) is too small and you cant wear it for long without getting a headache its shrunk to much over time. Get a can of Denatured Alcohol at the paint section of the hardware store, mix it 60% water 40% alcohol in a spray bottle. Spray the inside of the sweat band all the way around, on the inside of the hat where the brim and crown join. Then using your knee inside the hat, and one or both hands all the way inside the crown give it a good tug, you will feel the stretch, or better purchase a " hat jack " on E bay and stretch the hat with it while using the spray. It softens the material making it more elastic.