Can a long message create a crash? Is there a limit?
Not sure if there's a limit (seen boxes that fill 2/3 the window), or if it can cause a crash, but when you're writing something as long as a quest journal entry, you're almost always better off just making it a quest journal entry, or splitting it into 2 or more message boxes. Nobody likes reading a wall of text, and since message boxes don't allow much formatting, that's usually how it ends up.
If you're in doubt, you could always test with a temporary message that is shorter in length.