Oh, okay...
Take all your shit, go to REI or someplace that sells packs, put all your shit in the smallest pack it will fit in, see how big it is, and that's what she's talking about.
There really is no reliable average ... some people travel with ~20L school backpacks, some travel with large ALICE packs (which are ~75L, my personal estimation) with a bunch of shit strapped to the outside. It depends on what you carry.