how to choose the size of the?
ROBERT K If you have an adult sized head you want the largest size. Even then it will be a bit tight, but it will loosen over time (took mine a couple of weeks to get really comfortable).
You can speed up this loosening by putting the cap over a bent knee, hooking your hand under the edge and just stretching out a bit (the way you would do with a cap that you'd shrunk a bit in the washer.
Note that if you have an adult sized head and a lot of hair, even the largest size may not be large enough for you (just like the other speedo caps)