You should ask it in plugin's thread. I assume this one:
Aitum submitted a new resource: Aitum Vertical - Separate canvas to manage your vertical content Make content for TikTok, YouTube Shorts, Instagram Live, and more without the fuss. Use a separate canvas to manage your vertical content, and link your normal & vertical scenes together for...
obsproject.com
In general, if application's window min size can fit into the screen and window can be resized (programmers may design any window of the program as fixed size window), then you can right-click over the header of the window (context menu will appear) and click the
Size option, then you should press
Down Arrow button on keyboard (this internally selects bottom edge of the window for resizing), then you should press few times
Up Arrow button on keyboard (with each key press window shrinks from bottom to top), then you press
Enter button to finish resizing of the window.