Simple scan with relatively nice resolution to png format.

scanimage \
    --format=png \
    --resolution=300 \
    > output.png

Force US Letter dimensions. x and y correspond to mm. US Letter is 215.9 by 279.4 mm. Also, force grayscale mode.

scanimage \
    --format=png \
    -x 215.9 \
    -y 279.4 \
    --resolution=300 \
    --mode=Gray \
    > output.png

I then typically use pdfjoin to convert the page(s) to a pdf file. I often print these files via Google Chrome, and am sure to choose Black and white and to not choose Fit to page.

Sources