Do you know if the jump from 13 to 15 only increases the width or does it also increase the depth of the bag? Could you take a pic of it with something for scale by any chance?
Hey, I got home and tried a few shots. My ideal bag would probably be a little thicker (more horizontal space) but this bag definitely compensates with extra pockets and is extremely high quality. It feels rugged, even if you over stuff it. The problem I found is that to get an extremely large messenger bag, you have to get a flimsy one (they're almost like stitched grocery bags with a strap). I wanted something durable with good stitching and
padded pockets. Several laptop bags didn't have any padding, so you would bang the laptop around when you set it down, dropped it, or bumped into a wall. This one has proven relatively roomy and rugged with padded pockets.
All these shots were taken with my 15" MacBook Pro inside the bag. So if you didn't need that on you, you'd gain a little more space. They're not the best shots, but I have bad lighting and the bag is hard to hold with one hand while you take shots with the other and keep yourself out of the frame.
I posted a few images to help give you an idea. I used a 1" textbook, a 3" binder, a 0.5" binder, and a Cheeze-It box (why not!).
I'd say 2-2.5" is ideal. You can fit a 3" binder in there, but that's a max (a little tight too, but doable). What's nice though, is that you have many more pockets than just the main compartment. There is a large zip pouch on both sides of the cover-flap and a really large pocket on the back (see iPad shot). You could fill the compartment with binders and books and still be able to fit an iPad, notebook, or folder in that back pocket. The way it's made, stuffing the main compartment doesn't really make the back pocket unusable. There are also smaller pockets on the very front of the bag for smaller things likes a calculator, charger, iPod, pencils, etc.
My typical load would be my MacBook Pro, a 0.5" binder, a 1-1.5" textbook, and my iPad. This bag can hold that plus a little. And it's long enough so that you can fit something next to the textbooks like a charger, bottle, or a snack.
Edit: To clarify, all the shots of the bag next to a binder is using that large 3" binder
