I had this problem as well, however I have solved it.
I positioned Outlook on the right side of my monitor, and my new notes/emails/tasks to the left. I like to see both things at once. However, Outlook would always open the new email/note/event as a large, irregular window that overlapped the main Outlook window. Frustrating, because I like to see it as described. I would always resize it, use it, close and then test to see if it remembered the size and position. It would not.
So I moved Outlook from my secondary monitor to my primary monitor, and then repeated the positioning. Outlook finally remembered! But, I don't want it on that screen, so I moved it back to the secondary monitor, and to my surprise it did remember.
Problem solved (for me).