Packs are typically measured by volume (cubic inches in the USA). It's hard to measure them any other way because of their nonstandard geometry. Your best bet is to bring your current pack and find something that's of comparable size, just eyeballing it.

Another approach, of course, is to lighten your load. I keep toiletries in my desk at work, also computer accessories (and most days computer as well), and I carry a bike bottle instead of a big ol' camelbak.