A Side Kick Backpack is as useful as you think it would be. When there is nothing inside, the dimensions are: H 53cm X W 37cm X D 10 cm. There are zippers everywhere that have their own storage space for specific use and when I type specific, I mean specific to YOUR needs as you may place certain items into certain compartments.
Right at the top we have a zipper that opens up to a H10cm X W28cm compartment for any small items you have on you that you don't want to carry around. Right beneath it is a H10cm X W20cm compartment for your other bits and bobs. In the middle, the zipper opens to a compartment which could hold your magazines if you have any spares, 4 slots for pistol magazines and one for a M4 or smaller size magazine. The right side of the bag has its own zipper that opens up into a MOLLE panel so you can attach anything you want within that you deem not worthy to apply to your vest but still important enough to keep around.