How to make display fit external monitor

Hi there,

I have an external monitor connected to my Macbook Pro but the display doesn't fit the display size, if you see what I mean. When I make a window full screen I can't see the far left or right hand side (including the red and yellow buttons on the left hand side), and when there is no window full screen I can see a tiny bit off but not all of the top menu bar. It seems like the detected screen size is bigger than the actual screen size, so these bits and getting cut off. Is there a way to resolve this? Thanks for your help!