I have two monitors and one has much larger display than the other. How do I fix this?

One of my monitors has a much larger display than the other. When I try to go into settings, it says I cannot change anything because it is less than 600. Not really sure what this means.
Update: I'm not a computer person. When I had only one screen, the monitor would occasionally switch to the large display
Update 2: When I try to change the resolution I get this message: If your resolution is below 1280x960, some items may not fit on the screen.

Also, where I would change the resolution it is grayed out at 640x480.