Our birth plan was about three pages... but we did have just over a page of writing that explained our family history. (we lost our first DD to heart problems, so we wanted to spell out all of our feelings about that and how it effected us for the birth of our second DD)
I tried to put as much as possible into point form. Then what you want doesn't get lost in the mix, and it is more specific and easier to read. I took a few copies with me to the hospital. I printed it up on coloured paper, so when ever the midwives looked at my file, they all went to the coloured paper and wondered what it was... so they all read it.
Your birth plan needs to be as long as your list of requests. Of course if its is a very long document, then it will be harder for the midwives to take it all in. Throw EVERYTHING in first and then tweak it as you go.
Like some of the other girls have said, anything can happen, so just go with the flow. I think though, having a plan written out, makes you think about the kind of birth you want to have, and I think it can really help sort out things in your head down the track, if they don't go to plan. I think, even just writing it down, and make it clearer in our heads. Sometimes we dont realise what expectations we have in our heads, until we read them back on paper.
[Edited on 26/08/2007]