Gabe, most packs have a description of how much space is inside.
It could be described as cubic inches or in litres.
If you notice, a lot of motorcycle luggage like tank bags and tail bags are described that way. That's how a person judges how much something can hold.
Heavy-duty 480D polyester hex rip-stop
Padded laptop computer area
Lined music holder with headphone entry
Multiple outside pockets
Side drink holders
12" W x 18.25" H x 7.25" D