I'm between a medium and large size, leaning more towards large.  I saw reviews that this tends to run large, so I bought a medium and it fits well.  It's durable, light weight and works well when I'm out mowing and using the weed-eater.  this is a good hat to keep the sun off the ears and back of neck.  It's not floppy.  The string can be used to keep the hat on when windy or it can wrap around the back.