Macbook Pro 15 inch resolution on external 16:9 display issues

I just bought a Macbook Pro 15inch non-Retina display. I'm trying to connect it to an older Dell monitor I had that is 1600 by 900 resolution. I am trying to mirror the display but don't get a 1600 by 900 resolition when I look at my Macbook Pro display settings. I have a VGA connected to my monitor and macbook using the vga to mini display port. How do I get 1600 by 900 resolution? Do I need a bigger monitor?