Hi, your font problem is easy to solve : go to the font configuration part of system settings, and enable "force DPI" and select the font DPI you want. Or simply change the font's size.
To change your displaymanager, simply reinstall one, and you should be asked which one to use as default.
$ sudo dpkg-reconfigure kdm
For your login screen problem, check that you are using the correct driver for your graphics card, that you have the correct resolution... A screenshot would help.