XX,
If you do not have LogonStudio installed, or any other Logon editor, then check the video settings in the BIOS (Video > Enhanced or Enlarge or similar).
If there is no setting, try setting the BIOS settings to "Default", or "Optimized" to see if the Logon goes back to full screen (assuming that 1024 x 768 is full screen).
If you have LogonStudio installed, you might try restoring the default logon screen.