I used to think that a screen say the size of a 13 inch Macbook would be great, but after using an iPod for just a week; putting the Macbook in my lap made it seem huge like a 17 inch laptop. So now I think the iPad has the ideal size. As you get smaller you are going to lose functionality. Bigger and you lose portability.
me i think the Ipad unit is the perfect size for general all around use ..it just right for useing on the lap or a table ..for i really only use for less than a couple hours a day total when i do use it on a flat surface or on my lap or on the bed beside me ..so i not really holding it for long time frames .when i do use outside of those areas is rest on top part of the left arm and held againest my body by the action of the arm to use it like a oversized book unit to use it ..so i do not really notice the weight like that for i use it for short time frames to check books titles to see if i have them in the collection ..
but i do not use it as a e-book reader so iam not sure about that fuction of the unit in that typle left hand holding it to read from tablet set up on the unit ....
I'd love to see a whole range from iPod touch size to iMac size. Small is good, but so is big