Open Terminal
Type: sudo vi /etc/default/grub
Find the line starting with GRUB_CMDLINE_LINUX_DEFAULT, and add video=hyperv_fb:[the resolution you want].  
The resolution I want is 1280x720.  So my line ends up looking like this: 
  GRUB_CMDLINE_LINUX_DEFAULT="quiet splash video=hyperv_fb:1280x720"
  
Write the changes and quit vi.
Run: sudo update-grub
Reboot the virtual machine

Reference:
https://blogs.msdn.microsoft.com/virtual_pc_guy/2014/09/19/changing-ubuntu-screen-resolution-in-a-hyper-v-vm/