Ubuntu 16.04 has only one option after installation, 1024×768. Using the xrandr command has an error:

xrandr: Failed to get size of gamma for output default

You can’t set it using the CVT command, nor can you modify xorg.conf.
Solutions:

打开：/etc/default/grub
搜索：#GRUB_GFXMODE=640x480
编辑：640x480 改成你想要的分辨率，并取消前面的#
例如：GRUB_GFXMODE=1920x1080 
更新：sudo update-grub
重启: reboot

If the above method has not been solved, then there is another situation is the video card driver has not been installed, the need to reinstall the video card driver can solve the above problem.