Hi,
I had the same problem on my MacBook Pro. Outlook was fine until it wasn't. I don't know what changed, but last week, I started noticing the same issue as you described; I couldn't resize my Outlook window small enough to fit my screen, and switching to a full-screen view didn't fix it.
After reading the replies from others with the problem (the administrator's responses did not work for me), I switched to legacy mode and then switched back to the updated version to find that the new Outlook was fitting to my screen again. I wouldn't call this a fix, but it seemed to be the only way to reset things.
No idea if that will work for you, but I'm taking the win for now!
Cheers,
Jennifer