I've also got the Girls Backpack from the backpack/suitcase set. Vew-tori has already given dimensions, but as for carrying capacity:

It last carried 2 hardcover gaming books, 30 pages worth of character sheets, 1 large sketchbook, a box of pencils, and a day-planner. I've also carried up to 3 books and my viao in it.

(A gaming book is usually about 9"x12" and varies in width from 1/2" to 2", or the size of a narrow textbook)

While I like mine, I'm still not quite convinced it's worth the full $34. I got mine at the end of the Code Craze, and I do think it's worth the $24