To me, it shouldn't matter how long or short it is. You don't want to just say "Hey, I want your autograph.", but you also shouldn't send them a novel. What I would do is introduce yourself, give some information on what their work means to you, ask for the auto., and close it politely. Don't worry about the length so much as the content.

Good Luck!

Mike

How long is 'short'? You should ideally limit your letter to one page. All you need to write is an introduction, a body of 2 or 3 paragraphs, and a conclusion.

Anyone have good ideas on writing a letter?
I start writing a letter and I get done but then I notice its way to short
I'm not that good at writing letters as you notice so any help would be great
thanks a bunch!