Ponder this: Being DF'd or inactive is very different from being an Apostate.
this is true. being df'd means you are an unrepentant sinner, being inactive means you are spiritually weak. the end result is the same though: you will be shunned.
what it boils down to then, unstop, is how do you want those who shun you to see you/label you. If you don't give a rat's behind, go ahead and write your letter and call it a day.
to answer your original question, my letter was 5 or 6 pages long. looking back, I could have shortened it significantly.
Perhaps you find yourself coming up with more things to say because you are still subconsiously working out your reasons. That's fine.
Do this: write a little at time, stop for a while, then go back to it. Repeat that as often as necessary, as long as it takes.
Then once you are satisfied with having said everything you want to say, start proofreading and editing.
Lastly, before you sign and seal your final draft, wait a full 2 days before mailing it or handing it in. Re-read it, then go for it.
My 2 cents.