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've seen 7" and 10" tablets, and I think the 7" is best for portability, but the retina display 10" iPad is a beauty...

I do not think there is an ideal size it entirely depends on a users requirements. I have an iPad and would not want a bigger or smaller diplay but I do not want to carry it around very much.

A friend would love an iPad but requires the ability to have it with him virtually all the time so went for a 7" Samsung.

More important than screen size is form factor.

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

...let us know when the jury's in then:)