A too low resolution in Ubuntu after a default install can be fixed on the CLI:
sudo dpkg-reconfigure xserver-xorg
Just run the defaults with hitting the <enter> button , sometimes you have to highlight the ok with the <tab> button before hitting the <enter> button. When you see this screen:
Select your desired resolution with the <arrow-up/arrow-down> button, then hit <space-bar> to confirm.
Now continue with the defaults until done.
After restarting your X-Server (ctrl-alt-backspace) your desired resolution should be selected.
If its still not implied, look in your panel :
System > Preferences > Screen Resolution
The desired resolution can be selected there.
Also read this if you need more information.