Brother DCP-4020C - Manual (Page 109)

9 - 10 HOW TO SCAN USING WINDOWS
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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.
■ 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.
You can see the actual paper size you selected on the screen.