[Gretl-users] Plot Commands Editor

I am trying to edit some of the plot commands as described in the manual. Specifically, I am trying to change the canvas size for pngcairo from 640,480 (the default size) to 480,480 or some size close to this. When I make the changes as described in the manual (at least, as I interpret the manual) either nothing happens or I get dinging bells (which I am sure indicate some kind of error). Any suggestions would be greatly appreciated.