Yesterday, I watched my friend display some timetables / rota data on his smartphone screen and was very impressed how he could expand the photgraphs onscreen with finger and thumb...an ideal tablet screen dimension would be around that size...two-handed hand-held portable devices don't work.
I agree with millgate that 6" is probably the best size for everyday usage without having to carry it in a bag. My Kindle fits nicely in an inside pocket so it goes lots of places with me without any inconvenience. If I have to suffer the inconvenience of carrying a bag I would rather have the power,storage capacity and capability of running my usual programs without difficulty, that a reasonably decent laptop would provide. Obviously battery life is against the laptop choice but I could live with that rather than a 10" tablet.
Eight inch seems a good compromise, but more important than the size is the form factor. A 4 by 3 ratio as used by the iPad is much moe useable for most tasks than the 16 by 9 in common use elseware, which is better for watching films, but worse for everything else.
The 4 by 3 of the iPad and afew other tablets is very usefull.
The 16 by 9 of the majority is better for viewing films, but far worse for everything else.
The new Nexus is 16 by 10. A step in the right direction for a seven inch device.
I hope this is the start of a trend back to a more usefull form factor.
The wreched 16 by 9 is now firmly entrenched on Laptops, and you spend most of your time scroling to see what is being displayed