On the [E-mail settings] tab, set the maximum size for the image files to be attached to e-mail.
 

Maximum size of attached file
Select the [Display confirmation message if file size exceeds the above] check box to display a confirmation message when an attached file exceeds the maximum size.

• For Scan First, a confirmation message appears if the file size of scanned images is larger than the file size set here.
• For Select Scan Job, a confirmation message appears if the set file size is exceeded during scanning.