After installing Ubuntu 13.10 on Windows 8.1 running Hyper-V the guest's screen resolution is fixed at
1152x864. I found out that Ubuntu needed some services to run optimized as a virtual guest.
To fix this, I ran
sudo apt-get install linux-image-extra-virtual
which installed everything Ubuntu needs to run as a virtual guest.
edited /etc/default/grub so the GRUB_CMDLINE_LINUX_DEFAULT line reads:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash video=hyperv_fb:1600x900"
then ran
sudo update-grub
a reboot, and we're there.