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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].  
   
   GRUB_CMDLINE_LINUX_DEFAULT="quiet splash video=hyperv_fb:1920x1080"
Write the changes and quit vi.
Run: sudo update-grub
Reboot the virtual machin

See https://github.com/LIS/LIS3.5/blob/master/hv-rhel5.x/hv/hyperv_fb.c for details

 * This is the driver for the Hyper-V Synthetic Video, which supports
 * screen resolution up to Full HD 1920x1080 with 32 bit color on Windows
 * Server 2012, and 1600x1200 with 16 bit color on Windows Server 2008 R2
 * or earlier.