I think the Take screen shot activity has an added attribute to set the image width and height with reference to the top-left area of the screen. Here is the info from the documentation page:
Target.ClippingRegion - Defines the clipping rectangle, in pixels, relative to the UiElement, in the following directions: left, top, right, bottom. It supports both positive and negative numbers.
Based on screen resolution you can arrive at the width and height you want to clip.
In case the Target Clipping Region doesn’t work, you can still use the Screen Resolution to compute the proportionate clipping area of your Excel sheet.
You can compute the screen resolution at runtime as shown in this thread: