I have a 15" laptop with a 4K screen, i.e. with a very high DPI.
With the gdm3 display manager, the font size was OK by default.
But when I switched to lightdm, the fonts became tiny and difficult
to read. For obvious accessibility reasons, the fonts should be
large enough by default.
Note that if the fonts are too small, the user may not have the
ability to log in in order to change the settings. Typing F1 allows
one to enlarge the fonts, but the user can have this information
only via the menu, thus with potentially unreadable fonts.