External Monitor issue

I hooked up an external monitor to my macbook pro (early 2011) via: the thunderbolt to DMI conncection.

I have "so-so" resolution and there is about a two inch space on both sides. It does say it is 1920x1080 @ 60hz

However: I closed my laptop lid while it was on and just like that; the monitor filled the screen and the resolution is clear as glass....

anyone know how to display this resolution permanent??

I have a ViewSonic va2431wm....thank you