Vim autocomplete to store words between documents

User statement:

I use Vim to write many technical text documents, including emails.

Question:

Is it possible to "store unique words" from past documents in some .vim/file that would be automatically loaded when any future file is opened and the Ctrl+N command is issued? I know others have asked about pointing to a dictionary file in addition to the current file (e.g., Vim auto complete, both with current file and dictionary) but I'd rather the process was dynamic.

Ideal behavior:

I'd like to write one email containing Supercalifragilisticexpialidocious. Then, in some future email, I'd like to start writing Superc and then Ctrl+N would complete the word.

Further thoughts/requests:

1. I assume there is concern about such a "carryover file" growing excessively large over time. It would be great if it is possible to prevent "words shorter than K characters" from being stored.
2. I'm also willing/interested in manually editing such a file.