It is seen that some people like cow boy hat to be tighter while others like them to be loose but ideally ,it should have fit optimally right neither too tight nor too loose . It means it should not fall off as well as should not give headache.
Proper hat size can be calculated by using a cloth tape: for very first step take a cloth tape and measure the portion around your head where that hat would sit on and note it down. For example if your measurement was 24 1/3". Divide that value by 3.145 which will give 7.737. Now you can try size available closer to it. Normally most hat sizes ranges from 6 3/4 to 7 3/4. If you require a larger one than you might try to get a 7 ¾. Before moving to larger size stretched slightly if it’s only a fraction smaller but if you require even more space, you can also refer to customize options available and can go to a custom hat maker.
One more important thing that you must take care is that if you gain or lose weight then it also reflects around your cranium also. So, If you buy a hat and it's snug after some point than it may be the case you have put some weight if it’s the case frequently than you must follow following method to check it that at the time of purchase above the eyebrows it should fit about 1-2 fingers and above the ears it should fit around 1-2 fingers. And it should also be nearly parallel to the ground and should not tilt down on your head. That is it should be just tight sufficiently to fit comfortably and snugly and should not slip around.
But still if your hat pop off your head easily than it is a bad fit but don’t worry as it can be easily remedied. One recommended solutions can be any one of these. Use a hat string to tie the cowboy hat down to your head. Or you can also try this one, Get a wide foam insulation tape as used for weather stripping and now place its pieces inside the band. One of its side will be sticky so that it will stay in place after the application. This foam can make a snugger one to be perfectly fine and adjustable the great thing of this solution is that it is not visible and highly adaptable as per your specific needs depending on your workout.
Tip at the end:
After getting an optimal hat also maintain it in such a way so that it may not get loose for this Whenever you put on or take off hat try to always handle it the by the front and back of the brim using just four fingers. This will help to keep it clean and well in shape as it was when was very new. Whenever you take off it be sure that you place it upside down and always dust it off using it only a hat brush or a small plastic.