please note: large fonts on low boot screen resolutions can corrupt the boot screen
Because this problem depends on chosen font, it isn't possible to define a general maximum. When you're trying to select a larger font, please do it in small steps (rebooting and checking the results).
The problem occurs when the grub gui elements (borders, description) wont fit into screen. So while there's enough vertical space you can try a larger font. The mass of menuentries is not a problem - they are scrolled if required.
The corruption results in a endless loading time of grub2. You need a live cd to fix this:
* delete the font from (MOUNT)/boot/grub/unicode.pf2
* reboot your system (menu should show up with default fonts)
* open /etc/default/grub with root permissions and place a '#' in front of the line containing GRUB_FONT
* then run grub customizer to choose the font used before
