Pacsafe Travel Anti-Theft Backpack & Bag Protector Size Medium with Case

Pre-owned. Great condition with the case, padlock and keys. It's a stainless steel locking device for anti-theft. 

This is an older version marked as a Medium. Because of that I don't know specific liter sizing they now use. 

It measures 22" tall when flat. I used it to fit my 21" tall,15" wide, 7" depth backpack. So I'd say it fits up to a large backpack. 

It has loops that can be reached through and small stuff still grabbed out of your bag, but it keeps it from being just run off with and bigger stuff being pulled out like a laptop. In general is a very good deterrent as it can be locked around a pole or chair while at airport if you need to sleep. Or can be used to lock your backpack and helmet to your motorcycle. Very useful item.