How to set up mirrored monitors

I have a 24 inch samsung Syncmaster T240 as my computer monitor have my computer also plugged into my tv (Samsung 46"). Both are plugged in through DVI.

I want to have the displays mirrored (have the same image) but my syncmaster monitor always has the black bars on the top and bottom.

It is set to Monitor(1920 x 1200) and TV(1920 x 1080)

Whenever mirroring is off then the monitor takes up all the space and has no bars

is there any way to correct this so my shiny new monitor can take up the entire new monitor space?