I resolved following this link: http://www.ubuntuforums.org/showthread.php?t=20976 and adding
DisplaySize 338 203 # 1280x1024 96dpi in the first "Monitor" section of xorg.conf -- Unreadable Font Size for GDM Login/Password https://launchpad.net/bugs/36251 -- desktop-bugs mailing list desktop-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/desktop-bugs
