Brother MFC 8840DN - Manual (Page 62)

3 - 16 HOW TO SCAN USING WINDOWS ®
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Width: shows the width of scanning area
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Height: shows the height of scanning area
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Data Size: shows the approximate data size calculated in a
Bitmap format. The size will be different for other file formats such
as JPEG.
You can see the actual paper size you selected on the screen.