You can't from VNL. But you can resize the exported image file
EDIT: you can resize manually the BS window to tune the aspect ratio of your plot.
The exported image will have the same width/height ratio.
In a way, you can make this window larger or smaller to tune the size of the exported picture.
However, if you wish to have an exact size as in your case as suggest you to resize the exported picture with an image manipulation software.