I scan in full color because, for some odd reason, Abby FineReader does slightly better with full color than black/white or grayscale. In my experimentation, this worked best (from left to right):
(best) Color > Grayscale > Black/white (worst)
This could easily be a quirk with my scanner, so I would expect Grayscale to be the best choice for some scanners. I also only do this on pages with small text, such as you would find in a commentary or dictionary, where they crammed as much as possible on a single page in a small font size.
200 megabyte files for every 100 pages sounds about right for grayscale. For me, in color, it was 300-350 megs per 100 pages. HD space is cheap and it doesn't bog my computer down when running large files through Abby, so I didn't worry about it. And when I'm done with the files, I just move them to an external 2TB hard drive. Ellicott's commentary was 29 gigs scanned in PDF.
Josh