ideally you should edit your post and split off gdm and start a new post for that pls
I will continue this post for grub only
look at your settings you can edit with root powers for file=/etc/default/grub for lines
# The resolution used on graphical terminal
# note that you can use only modes which your graphic card supports via VBE
# you can see them in real GRUB with the command `vbeinfo'
Now you probably have never pressed C for commmands at the grub menu stage?
but all graphics cards in recent 10 years can handle 800x600
so (1) edit that size if you want smaller font screens
so I like to use mtpaint to resize images to that resolution.
so depending on your currrent image (target image) you want to display
as non-root run mtpaint and check what your target image is.
(3) once happy put that image with its correct image extension as per list
(4) as root run
Generating grub.cfg ...
Found background image: background.jpeg
Found linux image: /boot/vmlinuz-3.11-3.dmz.2-liquorix-686
Found initrd image: /boot/initrd.img-3.11-3.dmz.2-liquorix-686
reboot and see if it worked