It doesn't mainly depend on your monitor because most of to-day's
accept a lot of resolutions.
It depends on your video card and on which driver is installed.

1) Check wether proprietary driver is active. If not, Ubuntu will search
for it and propose you to install it.
I don't know how the right menu element is called in the English
release, but
translated from Fench it would be about :
"System/Administration/Peripherals Drivers"
When you open it, Ubuntu checks which video drivers are available
for your Graphic Card and propose to install the most matching.
After installation you check "Enable", Close the window and restart your PC

2) So you should find in "Sytem/Administration/" a new element
linked to your new driver, which name begins with the name or your video
card
(for example mine is Nvidia, I found "Nvidia X-server Settings")
But better not use it directly from the menu because it's possible you
won't
be able to save the parameters so that next time you boot up they would be
the same as before modifying.
What to do then ? Right cllick on the main menu
and confirm "Edit the menus". Then find what you noticed as new element
of the menu and right click "Properties". In the small window that opens
at the command line item, select and copy to the clipboard _the last part of
the line (just after last slash "/" till the end of the line,_ and save
it with a text
editor such as to keep easy access. Now open a Terminal, log as root
and paste in it what you copied + {Enter]. This way you call your settings
manager as root user and you will be able to change your screen resolution
and keep it changed until you don't decide to choose another one.
a) Apply b) confirm you want to keep the way screen is now displayed
c) Save to xorg.conf with the "Merge" option checked (better for not
changing
too many things at the same time ? sometimes set a mess)
d) Everything would be OK !

By the way, do you know how not to log as root each time you want to use
a Terminal ? Make a special shortcut for a "root" terminal and for
the command line enter "gksu /usr/bin/x-terminal-emulator"
So you just have to enter your password from time to time.
Just once if you don't keep it unused too long.