Do you have nvidia? And I'm guessing you've installed the nvidia drivers?
Open terminal (Ctrl-Alt-T)
sudo apt-get update && sudo apt-get install hwinfo
Pick one of the resolutions and bit depth that matches your native resolution, or at minimum the same aspect ratio.
sudo hwinfo --framebuffer
Copy the 6 character hexadecimal number. ie: 0x0361
gksudo gedit /etc/default/grub
(replace 0x0361 with your hexadecimal number)
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash vga=0x0361"
Save the file, close the file,
Then reboot, and see how it looks.
This is one way, and there are a few more ways to change the resolution.