Whilst running Ubuntu 20.04 in a Windows 10 Hyper-V VM, I found that I could not change the screen resolution. I am not sure what the root cause of the issue was but the following fixed it!
I can't remember why I needed this but the following is helpful:sudo apt-get install linux-image-extra-virtual
It defaulted to 1024x768, but 1440x900 works better as some dialogs are no longer cropped and it uses more of my monitor, to make the change, do the following:
sudoedit /etc/default/grub
sudo update-grub
sudo reboot