Ive been a digital painter for around 8 years and love the size of the 6x8. Im still using an intuos 1 6x8, before that I had a 4x5 graphire and that was too small to draw but still totally useable for painting.
I wouldnt want to go larger, larger=more arm work, and the 9x12 is freaking huge. I dont really have the desktop space for that monster anyways (9x12=active area, theres still several inches of extra space on the edges making it very very large). I havent used the 6x11 but I imagine those are a good size too, Im happy enough with just using widescreen proportions on the 6x8 though.
I have the 9x12 and it's just a matter of adjusting the area on the tablet to match your screen or even better you "can" adjust the area on the tablet to be smaller as that will match your screen corners as well thus less arm movement. I do find the 9x12 much nicer than the 16x24 sketch pads required in figure drawing at art school, now that was arm movement