I hace A Dell U2713HM screen that is 2560x1440 capable, but my Mac Mini is only 1920x1080 capable. How can I force the mac to display its output at 1:1 on the screen without scaling, so the display will remain sharp (even if it will be displayed smaller) ? I do this on my PC into the nVidia drivers, but I don't see where it can be done on Mac OS.
Go to System Preferences (the square gray icon with gears on it) in the Dock and choose Displays, then select the resolution from the scrolling list of possibilities. You should certainly be able to set 1920x1080, though of course there will be wide black bands around the output.
Generally, on OS X, unlike Windows, video hardware manufacturers do not install their own control panels or preference apps.