Clicking on the number (40 by default) to the right of where the New Art menu says "Width:" does nothing. Clicking on the empty space to the right of the number also does nothing. Clicking underneath the number, in the space between the number field for Width and the number field for Height, selects the Width.
The same is true for the Height number field. The cursor has to be placed underneath it to select it. I've attached a diagram to be more clear. This is on playscii version win32-0.9.6