Ubuntu and Hyper V don't always play nice together in the graphics department. You can edit the resolution via grub
Open grub conf
sudo nano /etc/default/grub
Edit line
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash video=hyperv_fb:1280x720"
Update grub
sudo update-grub
Powershell. This one didn't work for me from stackoverflow
set-vmvideo -vmname "Ubuntu 20.04 Download" -horizontalresolution:1900 -verticalresolution:1200 -resolutiontype single
Done like that