The 8.1" DR800SG isn't much bigger than the Kindle 2:
Code:
Hanlin A9: 262 x 170 x 12.0 mm (10.3" x 6.7" x 0.5")
DR800SG: 193 x 149 x 10.7 mm (7.6" x 5.9" x 0.4")
Kindle 2: 203 x 135 x 9.0 mm (8.0" x 5.3" x 0.4")
Nook: 196 x 126 x 13.0 mm (7.7" x 4.9" x 0.5")
PRS-600: 175 x 122 x 9.7 mm (6.9" x 4.8" x 0.4")
Minimizing the overall device size is a potential advantage of a touch screen, and presumably a 6" device relying more heavily on touch could be made smaller than the PRS-600. A SiPix 9" screen is 15 mm wider and 20 mm taller than the Vizplex 8.1", so perhaps 8.5" x 6.5" is the smallest practical size (the A9 has a D-pad ring below the screen which makes it taller than this).