Using Classic Outlook on Windows for personal email, calendar, and contact management
This is annoying as heck. Luckily, the fix is simple. You have to set the size you want in what I will call the "prime" window. Do this:
1. Close out of Office completely.
2. Open back up. Hit tab to shrink from full screen.
3. Change the size of that window to the size you want.
4. Close out of Office again by Ctrl-clicking the X
All new non-full-size windows should now open in the correct size.