The "Form" tab in the local extension allow you to specify a default width and height for the current window. When you enable this option, prior to capture the window for printing, the window will be resized to the width and height you specify here, and after capturing, it will be restored to the current size.
The user might note the resizing, although it happens very fast.
The purpose of this feature is to avoid the window distortion when you allow the user to resize it. If your user may resize the window, and it "distorts" it (change the width/height ratio) prior to print it, it will be in fact printed with the same distortion. If you need to avoid the distortion (for example, when you are printing a pre-filled form with a graphic background), you may use this option.