Yes, it will fit in a backpack. It is about 8 inches x 5 inches x 3 inches, and weighs about 3 lbs. It does have to be plugged in to electricity, though, so it is not suitable for use where there is no electricity. The speakers that I looked at that were made for portability were much smaller than this one, and would fit much better in a backpack and not be as heavy to carry around, but I don't know how much the sound quality would suffer. I listened to the little Bose Mini speaker listed below, and it sounded pretty good in the store.
Bose - SoundLink® Mini Bluetooth Speaker II - Carbon
(3,399)