To set the width of the shadow first get into the edit mode and click on the pencil icon. Next click on the style tab and choose the shadow icon. Then, choose which edges and click on the edges button. Next, choose the color from the drop down menu. Finally set the offset or width by sliding the slider or typing in the number of pixels you desire.
The width of the shadow is referred to as OFFSET. So if you want to set the width of the shadow you have to set the offset property. The offset is also the depth of the shadow which can be defined by number of pixels.