Using the New Outlook for Mac for personal email and calendar
Yes this is exactly my issue! The quick fix from the Microsoft person to "Use macOS Split View: If Outlook is not allowing manual resizing, you can use the macOS Split View feature to have Outlook and Teams side by side. To enter Split View, hold down the 'full-screen' button in the upper-left corner of a window, then choose which side of the screen you want it on." solved my issue enough to make me happy again being able to have a split screen, but none of the other solutions fixed the actual issue of not being able to resize the screen manually if I want to. See the response above, maybe those other solutions will help you? Good luck!