Custom Dimension rendering

I did a search for it and didnt come up with what I had hoped for. Im trying to do a render with the dimensions 1900 x 2800 for something im working on. I tried typing it into the Format tab and all that came out was a 800 x 600 picture still. Any feedback please?

Works for me on v2.45. What version are you using? And where are you entering the dimension?