This is what it says in the book:
"Philosopher's Wool yarn washes easily and feels much softer after the first washing. Our sweaters also grow 2" to 4" the first time you wash them. Because our wool has not been denatured by chemicals, acids, and bleaches, each fiber relaxes and elongates as the oils are washed out. this means that if your sweater is a bit on the snug side, you can easily make it fit more loosely by washing it." Then it goes on to give washing directions.
I suppose you could knit a smaller size and wash it, but it still makes me a bit nervous. I love their colors, though.