Once Ubuntu is installed in Hyper-V login in and open a Terminal Session

In Terminal type: 

sudo nano /etc/default/grub

Find and change value of 

GRUB_CMDLINE_LINUX_DEFAULT with below. Replace the 1920x1080 in the commands with your highest resolution possible.

So it becomes something like this

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash video=hyperv_fb:1920x1080"

Save the changes and then run the commands,

sudo update-grub

sudo apt install linux-image-extra-virtual

Now you need to shutdown the VM and open Windows PowerShell as administrator on host machine and run below. Replace "Ubuntu" with name of your virtual machine. 

set-vmvideo -vmname Ubuntu -horizontalresolution:1920 -verticalresolution:1080 -resolutiontype single

set-vm "Ubuntu" -EnhancedSessionTransportType HVSocket

Start your virtual machine and open the view settings - Zoom Level and choose scale 200.

Open your Ubuntu Virual Machine full screen and it should be working just right.