What does this Sublime hat size mean?

Sublime hats have a soft construction, without a stiff headband, so their sizing is more forgiving and they choose to use a different convention than standard hat sizes.

For the most part, 0 is about a 57-58 traditional hat size, 1 is 58-59, and 2 is 60-61. 

If you find that you have an average size head, go with size 1 and adjust from up or down from there.