When i enter the game i just get a small boc of the game in fullscreen mode like a 600*800 box in the middel of the screen even through my res is set to 1680'1050

That feels like a really strange resoultion. Have you tried setting the game to some other resolution.

Another thing is that your screen could be too small (or not the right with/height ratio) for that resolution....why the game does that probably...

...or technically it is probably the system doing that...

In menu it's normal.
If it happens during gameplay, change scaling mode under your GPU control panel (NVidia Control Panel, ATI CCC) to 'maintain aspect ratio'.