In some situations your Mac may get to the log-in screen and display it at a different resolution than when it logs in to the desktop. This may result in the log-in window being fairly big with large text, and the screen blinking to black when you log in while it changes resolution.
This problem happens because the system stores resolution both in globally accessible areas as well as for individual user accounts so users can run at different resolutions and not have to change the settings at each session. Sometimes these settings can either get stuck or improperly set, especially if you change monitor outputs frequently (ie, attaching projectors, switching between TVs and computer displays, or using multiple monitors).
To fix the log-in screen's resolution, try logging in as administrator, changing the resolution of the display, and then changing it back to the desired resolution. If this does not work, try resetting the computer's PRAM, and then changing the resolution again. The PRAM holds some resolution settings that take effect at boot-up.