Open a Terminal (Ctrl-Alt-T)
Run
sudo vi /etc/default/grub
Find the line starting with GRUB_CMDLINE_LINUX, and add
video=hyperv_fb:[the resolution you want]
If the resolution I want is 1280×720 then my line ends up looking like this:
GRUB_CMDLINE_LINUX="… quiet splash video=hyperv_fb:1280×720"
Write the changes and quit vi by hitting ESC and typing
:wq
Run:
sudo grub2-mkconfig -o /boot/grub2/grub.cfg
or when installed in EFI mode
sudo grub2-mkconfig -o /boot/efi/EFI/fedora/grub.cfg
Reboot the virtual machine