The file size for created documents can vary a lot but generally speaking if you
want small documents that will be fast to email then scan in Black and White
and do not use a high dpi, 200dpi should be fine for most uses.
A Color or Greyscale document will actually be about 10x bigger then the
equivalent Black and White document but you can set the compression for Color
& Greyscale in the settings dialog.
You can set a value of between 10 to 100 where 10 is the smallest file size but
the least quality. 70 is the default setting.
1 page scanned at 200dpi Black & White will be about 25kb but would be over
200kb for Colour or Greyscale
Tip: When trying out different compression settings just click the PDF preview
button and you can see the results without having to save the document first.