> The smaller the device, the better.

Be aware that the thin scanners use different technology (CIS)
than the thick ones, which use CCD.  Word is that the thick CCD
ones give better image quality than the thin CIS ones.  For me,
the height difference isn't a problem.  A scanner that can scan
A4 size paper is going to use up a bit more than an A4 area on
your desk/table regardless of the height.

Be sure and check out sane's list of supported scanners.
For example, many Epson scanners have good support, but some Epson
models use a different engine and don't have the same level of support.

I have an Epson Perfection 2400 Photo.  Sane and NetBSD 1.6.2 both
needed some minor mods to get things working, nothing major.
PR 26117 has the minor fix for NetBSD.  Still open after more
than a year.  Has anyone even looked at this PR?