3.5". Or 10" as what Apple put in the iPad?
Dell's new Android based slate has a 5" screen. Barnes and Noble's Nook offers a 6". So what's the perfect size?
I suppose it depends on what you want to do with it. The iPad offer a good size for viewing documents and the Internet. And as with any video, the bigger the screen the better.
Plus, it offers a semi-full sized keyboard for two hand typing. However, it does take some getting used to. In the beginning, I found it not as easy as typing on the iPhone with my thumbs but it did get better over time.
So it brings me to thumb-typing. I think for folks who complained about typing on a touch-screen, it probably works better then the screen is bigger. I think the ideal size is 4-5 inches though I'm doing quite well on the 3.5" on the iPhone.
At the end of the day though, the onscreen keyboard does play a role in how well the typing experience goes. I simply am unable to type as fast with the Android keyboard. From verion 1.0 to 1.5 to 1.6 to 2.1, Google has made noticeable improvement. Supposedly, 2.2 is even better but even Google engineers say they've got a lot way to go.
Having said that, my experience typing on the Nexus One has lead me to believe that 5-7" screen size would be optimal for a user who want to thumb-type no matter of the OS platform used. Plus, a tablet or slate that is 5-7" should wait much less than the iPad which weighs in at 1.5".
I'm sure we'll see a wide range of screen sizes. Pretty sure we'll see ones bigger than 10". It's a matter of balancing the user's needs.
I'm happy with the 10" iPad because of the large screen that works for both productivity as well as entertainment. Eventually, I think I'll probably have with a smaller size tablet to serve as my mobile communication center when I don't find the need to take the iPad with me. I would be fine with something that has a 5" screen.
On balance, I would be looking for a 7" tablet because it can as a mini iPad that can fit in my jacket or a small messenger bag.