This is done using the Minimum File Size drop down in the Blank Page Deletion section of the Image Enhancement step of the Job Settings Wizard. This setting uses the image file size in bytes to determine which files are blank pages.
In this box you would put the number of bytes for the threshold for which documents are considered to be blank, which will be deleted and those considered non-blank that should be kept. If the scanned or imported image file is smaller than the number in bytes for the file size then it will be deleted and if it is higher it will be kept.
For 200dpi, compressed, black & white TIFF images (the default format used by SimpleIndex) this is usually around 2500 bytes and 300dpi, compressed, black & white TIFF images (the default for OCR) is usually around 7500.
Depending on your scanner settings and how much black is in your images, blank pages could be significantly larger. If you are scanning small pages the margin for error will be less. You will want to do some trial and error testing to ensure the setting is right for your images.