I think the divide with form factors is: does it fit in your pocket? 3-4" phone? Yes. 10"? No. 7"? Still no.
I believe there are so many more use cases for 10" that Apple didn't just pick that number out of a hat. For example, I own a Sony Reader (6"). Great for reading novels. Terrible for technical books (and, by extension, textbooks).
I can hold my iPad on one hand. Thing is though, I don't. I really don't know what the use case is for holding it one handed. It's always sitting on a table or desk, on my lap or on my chest (if lying down).