Size matters.
2.8" WinMo 6.5 cellphone--too small for comfortable reading (Samsung SCH-i760)
3.2" B&W Pocket PC PDA--one of the best ereaders ever (Compaq Aero 1550)
3.5" color PDA--fine for reading (HP iPAQ & Dell X51)
5.0" reflective LCD ereader--bigger, better in some respects, and more pleasant than the 3.2" PDA (jetBook)
6.0" eInk Pearl ereader--at the upper size limit for pleasant use, although the Pearl screen makes a big difference as well (Kindle 3)
7.0" color ereader--too big and heavy, I took it back (Nook Color)
Based on my experience above, I'd say the size range for pleasant reading is 3.5" to 6", with 5" about in the sweet spot. YMMV
Regards,
Jack Tingle