Brother MFC-8420 User Manual page 218

If you selected Custom as the size, The Custom Document Size
dialog box will appear.
Type the Name, Width and Height for the document.
You can choose "mm" or "inch" as the unit for Width and Height.
You can see the actual paper size you selected on the screen.
Width: shows the width of scanning area
Height: shows the height of scanning area
Data Size: shows the approximate data size calculated in a
Bitmap format. The size will be different for other file formats such
as JPEG.
HOW TO SCAN USING WINDOWS
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