I’ve got as far as

Power down the VM and run in powershell:

> Set-VMVideo -VMName 'randonomicon' -HorizontalResolution 2560 -VerticalResolution 1440 -ResolutionType Single

Start the VM, open a terminal, and run this command:

  sudo vim /etc/default/grub

change

GRUB_CMDLINE_LINUX_DEFAULT="quiet"
GRUB_CMDLINE_LINUX=""

to

GRUB_CMDLINE_LINUX_DEFAULT="quiet video=hyperv_fb:2560x1440"
GRUB_CMDLINE_LINUX="quiet video=hyperv_fb:2560x1440"

(size must match in all three places)

Run these commands to configure the boot loader:

sudo update-grub
reboot

This seems to work for me.