grub2 isn't really fit in the best screen resolution that my laptop can support (1280 x 800).
How can I do this without using a third party application ?
To do this safely requires two steps.
Reboot and press and hold Shift to display your grub. Press C to enter console mode. Then type (for Ubuntu versions before 18.04):
$ vbeinfo
For Ubuntu 18.04 and later:
$ videoinfo
This will display various stuff how grub recognizes your display. At the bottom is "preferred mode" - in your case it should say 1280x800. 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.
Press Esc to return to grub and press Enter to boot.
Reach for your terminal and type
$ sudo nano /etc/default/grub
find the line
#GRUB_GFXMODE=640x480
remove the # and change 640x480 with the preferred mode you wrote down. E.g.:
GRUB_GFXMODE=1280x800
save, then type
$ sudo update-grub
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.
