It's like a large briefcase, intended for commuting but not touring, etc. The website says it is 30 x 34 x 14 cm or 12" by 14" by 6", but the 14 cm/6" is a bit of an exaggeration. The cavity is about 7-10 cm or 3"-4" wide at the edges although perhaps it could bulge to 14cm/6" at the middle, so there wouldn't be much room to cram a water bottle beside the middle of a laptop without causing a big bulge, but it would easily fit at the end if you had a small laptop. Yesterday I carried documents totalling about 125 pages, a minipump, an underseat tool bag that I threw in the pannier instead of strapping under the seat, a nylon rain jacket, some pants (not shown), a heavy wheel cable and a light seat cable, the pannier's shoulder strap and a couple of lights, and some other odds and ends, and I could have crammed in some more stuff. If you really overstuffed it so it bulged, the velcro'd flap might not stay closed.

Okay, just did a test and with the laptop (a small lenovo x61) in the sleeve, I can still fit the rest of that stuff except the seat bag in the pannier. However I actually prefer to carry the laptop messenger style, in a separate small shoulder bag when I take it to work. I think it gets less jolting from potholes that way.