The iPhone works, because it's small enough to hold with one hand, while you type with the other.
I can't see anything much bigger working TBH. At some stage, you need a keyboard to type on, and to hold the screen at a good angle. And then it's not portable so you may as well have a netbook/laptop.
I typically move my 15" ThinkPad around by grabbing about (checking...) four inches of the corner with one hand. I admit it's a strain to hold it up and type with the other hand, but a tablet would be lighter and more compact. The ThinkPad is heavy and has a long lever arm.
I can't see anything much bigger working TBH. At some stage, you need a keyboard to type on, and to hold the screen at a good angle. And then it's not portable so you may as well have a netbook/laptop.