I did a fresh install of Ubuntu 11.04 on a HP 550 Notebook. But grub2 isn't really fit in the best screen resolution that this Notebook can support (1280x800).
How can I do this without using a third party application ?
To do this safely requires two steps.
Step 1: find the preferred mode
Reboot and press and hold
this will display various stuff how grub recognises your display. At the bottom is "preferred mode" - in your case it should say 1200x800. Note down the value.
Note: sometimes, some buggy video cards incorrectly give Grub the wrong preferred resolution - if the preferred mode is much higher than you were expecting, then select the nearest mode in the list displayed that you were expecting.
Step 2: Setting the resolution in grub
Reach for your terminal and type
find the line
remove the # and change 640x480 for the preferred mode you wrote down.
save, then type
NOTE: the preferred mode has to be among those listed by vbeinfo. For example, if your preferred mode is 1920x1080 (a common 16x9 aspect ratio setting), your preferred mode is NOT supported by vbeinfo and may not work correctly. In fact, there do not seem to be any 16x9 modes supported by vbeinfo, as of Ubuntu 13.04. In that case you could try falling back to something common like 640x480, which, it seems most monitors support and vbeinfo supports. Also, not all the modes supported by vbeinfo are necessarily supported by your monitor and you may have to experiment.
Easy to use Grub2 settings customizer. Go to "Preferences" -> "Appearance"