what is ur screen resolution?make the dpi correct.for eg:in my 800x600@85hz display it is fonts size=96dpi.In Gnome(latest),u can set font size by going to menu
System>Preferences>Appearance>Fonts>Details>Resolution=96dpi for a 15" display.
You can tweak for ur display by following below post:
*www.thinkdigit.com/forum/showpost.php?p=158983&postcount=56
^only change is dont edit all those gdm files,just change System>Preferences>Appearance>Fonts>Details>Resolution =xy dpi
I found a better guide here:
*wiki.archlinux.org/index.php/Xorg#Display_Size.2FDPI
BTW,for few having gdm fonts too small,editing theme xml(.usr/share/gdm/themes/urtheme/urtheme.xml solves the problem.for eg:each field like passwd field where it is "pw" in .xml file.