Hi there
Some monitors (LG, Samsung etc) also need you to go into their HARDWARE Menu (use the remote if it's a TV type of monitor) and select the nearest equivalent to "Automatically adjust screen size".
Your picture should then re-size nicely to fit square (??) / centrally in the monitor without any aspect distortion. Typical HD size 1080p screen will be 1920 X 1080. If your video card supports higher then ensure that you don't set the resolution HIGHER than the MONITOR hardware allows.
Note if you are using Multi-monitors you will only get the resolution of the LOWEST monitor if you are using "Extend" feature. If you are using a laptop on the whole it's better to set the display to "External monitor only" as laptops often have a measly 1366 X 768 native screen resolution (or even lower) although they can display much higher resolution on an external monitor.
As the previous poster suggested - ensure also your Video drivers are up to date -- there's a load on the INTEL site if you have INTEL HD graphics - a lot of laptops have these chips. - Even on an ancient Sony VAIO laptop which has a native resolution of 1366 X 768 I get full 1920 X 1080 on an external monitor via HDMI and higher via the RGB interface (although then for sound you have to use the Audio out). 1920 X 1080 is fine on a 24 inch monitor.
Cheers
jimbo