It was kind of silly. I installed Debian 13 in hyper-V, just to see how different it is from Ubuntu.  Like Ubuntu, it starts in a default size. I looked in the settings and there was a Displays there so I tried it and got the screen resized to full screen size.

So I then went back to Ubuntu and tried that, and it worked. I believe that what helped was having guest services ticked in the Hyper-v settings.