I have seen as short (and useless) as 2 sides.
With your Tribunal tomorrow, I would avoid repetition, and then avoid adding in lots of examples of something just pick the best one or maybe two where it helps to clarify. Eg if it says 'high level of tantrums in response to unexpected change' then it is fine to say 'from 2 to 5 per day'.
Keep in mind that it has to work as a useful document in the school, at least at annual reviews, so if there is a paragraph of detail that can be summed up in a sentence then it may be worth it.
Also Part 2 has to tick all the boxes of including the difficulties that give rise to the provision in Part 3. It doesn't need to be a life story. I know they have to put in stuff about normal pregnancy etc, but I don't think it adds much, so you don't need to wax lyrical about development.
Part 3 is just a cut and paste of all the recommendations from the experts reports, although you can add your own if they make sense - eg if the LA has allowed something in part 2 that is educational, but made no provision in Part 3.
The real difficulty I find is in leaving in a tactful amount of the LAs statement. On occasions I have helped with this stage, I keep thinking how much easier it would have been if the parent had drafted the statement in the first place......
Good Luck tomorrow