I don't really have a particular length, however, I like to keep mine on the shorter side and not over-explain things because mostly people will either misread it, misinterpret it, or just skim through it. Therefore, I plan to keep mine within a page, or two pages at maximum, just highlighting my reasons and some explanation (brief and direct), and then give some closure and peace afterwards. I do plan to have multiple notes, for family, close people who have influenced my life, and then maybe a few short appreciation notes to people who have been a big influence on my life.
As for your note, do as you see fit and since you already written it, I don't really suggest trying to delete it or over modify it, just keep what you have. If you want to condense it, like @Walilamdzi said, you could summarize it and have that version as well, it's all up to you.
