On some occasions the text typed into login screen prompt boxes is either very small or very large - out of proportion to the rest of the display.
In addition the login screen is displayed at the wrong resolution and or depth i.e. as though these settings in xorg.conf are being ignored by the Display Manager.
To-date this has been due to an incorrectly configured xorg.conf file.
I have a Radeon graphics card. Running 'X -configure' the driver chosen, configured is Radeon (xserver-xorg-video-radeon). With this driver I get the issues described above.
The Desktop sizes fine after manually configuring the screen resolution.
The solution, as stated earlier, is to use the correct driver as opposed to the generic one. In this case the correct or better driver is xorg-driver-fglrx.
Change driver in /etc/X11/xorg.conf
Original Driver chosen by the 'X -configure' command was radeon.
Alternative to 'X -configure' to generate the xorg.conf
aticonfig --initial --input=/etc/X11/xorg.conf
If you cannot get hold of the right driver you can still improve things by changing which fonts are used where.
Identify which greeter theme you are using
$ grep -o 'GraphicalTheme=.*$' /etc/gdm/gdm.conf
Non-xdmcp i.e. attached logins use 'gdmgreeter', xdmcp logins i.e. remote logins use 'gdmlogin'
Theme based control/formatting files are .xml files. For 2.20.10 they are under /usr/share/gdm/themes/<theme>/ and referenced in the *GdmGreeterTheme.desktop file in this directory.
Identify the gdm greeter theme's .xml file
$ grep -o 'Greeter=.*$' /usr/share/gdm/themes/Human/GdmGreeterTheme.desktop
Edit the file accordingly
<!-- password box -->
<pos x="50%" y="47%" width="box" height="box" anchor="c"/>
<item type="entry" id="user-pw-entry">
<normal color="#000000" font="Sans 11"/> <!-- default font size used for
user and password input -->
Back up the original and the modified files as they will be overwritten on updates.