So, looking at these:

1.) Chrome: Welded Rucksack
Welded Rusack Backpack | Waterproof Backpack | Chrome Industries
13.5” wide, 18.5” high, 4.8” deep


2.) SealLine: Urban Backpack
SealLine Urban Backpack - Small - Free Shipping at REI.com
15 x 18 x 4 inches


3.) Ortlieb: Transporter
http://ortlieb.com/_pdf_en/transporter.pdf
15.4 x 19.7 x 9.1 inches


4.) Ortlieb: Velocity
http://ortlieb.com/_pdf_en/velocity.pdf
11 x 18.5 x 5.9


5.) Osprey: Pixel
Pixel - Osprey Packs, Inc :2014: Official Site
13 x 18 x 7


6.) Mission Workshops: VX Sanction // 20 Liter
// Advanced Project // VX Sanction Rucksack || Mission Workshop
12'' x 18'' x 5''


7.) Mission Workshops: The Rambler
Backpack / Expandable + Weatherproof Rambler Roll Top Bag || Mission Workshop
13'' x 19'' x 5''