This afternoon I've done a little research into radicchio to see when I should harvest. One site informed me I could start harvesting leaves anytime. The head can be harvest once it starts firming up; you don't have to wait until it gets to a certain size. Heat could spur the head to begin bolting. We've been getting hot weather this week, so I should watch for that.
I didn't realize it was a perennial, so hopefully it will come back next year!
Here are the links I thought most helpful: