Here are a few pictures of the Nook Simple Touch Reader. You can see the size of it compared to a normal old ink pen. And then in the palm of my hand. (Fyi from my wrist to the tip of my middle finger is about 8 inches). And the back of the Nook. Slightly indented for easy grip. And its sorta has a rubbery texture to it to prevent slipping.
The box says its a 6" screen (which is diagonal) top to bottom is about 5" and side to side is about 3.75".
but like i said. With the ability to change font size (PDF and EPUB) and the font itself (EPUB) it makes it easy to find the perfect size for you. *note: i have only used PDFs and EPUBs (i cant remember what other formats it accepts. And a picture of the box just for fun! :D