Using the New Outlook for Mac for personal email and calendar
Switching to legacy allowed me to size down, and then the smaller window size was retained when I switched back off legacy. I tested if I would be able to resize properly after switching on/off legacy, and the bug persists—I still can't get Outlook to a smaller width, only wider. It's good to know that there's a workaround, but it's a poor workaround that requires making settings changes and restarting the app twice... especially since my Outlook has spontaneously resized to the larger window multiple times, requiring me to do the workaround multiple times.