On the one hand, shorter the better is the best advice. Especially if this is the introductory information. The comments about people not ready, not wanting to read or emotionally unable to read all that you've written are going to be true of many.
On the other hand, I had to chuckle when I saw the heading of this thread
I just finished what is the equivalent of Gone With the Wind or War and Peace. The letter that I've written is for my three adult children and will be given to them AFTER I've spoken with them. It encompasses a long background of my childhood, what made me how I've been, why my two marriages have been difficult at best - with the huge exception of my children, and hence why it is only a month ago that I actually realized I'm transgender.
At a full 5 pages (12 pt), it will hopefully serve to fill in for all that's lost, not heard, misheard, not explained clearly or not at all when I talk with them either in person or perhaps by FaceTime (to compensate for communicating between Northeast US and the deep south). Even at that, I probably ought at least divide it into chapters with headings so they can ignore what they want - any or all.
I'm NOT recommending this for anyone - certainly not the length! But I feel I need to explain some of the stuff that has hidden my identity from me all my life. If I weren't so darned old, it wouldn't have taken that many pages!!
Still, I'll edit...and hopefully not add more!
And back to the original message... short is best.