The screen's physical size makes a difference as it also serves as the keyboard. And the smaller the keyboard, the less likely you are to type lengthy text. So an iPod Touch / iPhone is fine to type a tweet but is not very appropriate for typing a whole wiki page (say). The iPad's size means the onscreen keyboard is quite useful as compared to smaller devices.
PS: Steve Jobs jokes that devices with 7inch screens will need to be sold with sandpaper to make your fingers smaller to use the touch screen properly:
http://www.maximumpc.com/article/news/steve_jobs_says_your_fingers_are_too_fat_7-inch_tablet