Any hats bought to be worn during the summer season should fit slightly looser on your head for a couple of reasons...
1. A Panama hat will shrink a little bit over time due to the natural moisture, and heat. 
2. You will perspire in the summer and it will not be comfortable. 
3. A hat that fits snug on your head would leave a red print on your head, which would not be pleasant if you take off your hat. 
If you are living in a country with strong winds, you may want to consider getting hats with an adjustable velcro strap that can be tightened in the event of strong winds.
  • To measure your head, take a soft tape measure and place it at the back of the head just below the bump.
  • Wrap it around your head just slightly above your ear and meet the two ends in the middle of your forehead.
  • You may want to loosen the tape measure a little so that the fit is not too tight or you'll end up with a tight hat. 
  • e.g. If the number measured states a 58.4cm, you would consider a size 59 for your hat.