In your opened file of /etc/default/grub , mine shows a few lines like this, so find the similar part of yours
# 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'
# CRAP Obsolete use videoinfo
Note mine has the additional comment starting with Crap, just my gesture of defiance
Almost every Linux Distro known has this line, and it is obsolete. Manjaro is one of only a handful that have it right. So
3. When you boot to your blue grub menu, at the bottom will be a line or two, choose the one 'c' for command, it will drop you to a black and white screen with grub prompt.
There, you can try vbeinfo, it will likely tell you the command is not found.
Type in and enter
and it will list resolutions available to your monitor or laptop screen, with the one in use asterisked. Mine looks like this
You can see mine is on 1920 x 1080.
Take a list or a phone pic while you are there if you like, it can be handy.
Exit by pressing escape back to grub menu or entering reboot. Then
4. The lower the resolution, the larger your font will appear in your grub menu.
So in /etc/default/grub , a logical starting point is with the line
Just delete the # comment at the beginning to try that and you can always experiment further.
5. Save the file, and always after doing so run
before rebooting, or it will not take effect until you do.
BTW as well as the Grub menu, size of your splash icon, if any, may be enlarged, but your desktop session will not be affected.
Let us know how you go, and enjoy your Linux.