When I sent my letter, I just included three pictures I wanted to get signed, and asked if she'd mind signing them. There was another signed promo picture in the envelope I received back too, in addition to the three I sent that had "Leonie, Best wishes, Keira Knightley" on them.
I'd just write about why you think she's great: talk about Keira movies you really liked, and what you thought was so good about them... things like that. Explain why you're writing, basically.
I wouldn't make it too long: the odds she'll read it decrease with every page, I'd say. I wrote about a page and half, but that was back in 2003 or something, when she wasn't so busy yet.