I just used a Quicksilver backpack. Bought it at a Gander Mountain. Used to have a BugOut backpack. I ride with one almost every time I go out.
The BugOut Gear backpack was awesome. It could hold a lot of stuff. I put my visor, rain gear, camera, groceries, couple two liters of cola... you get the idea. Had that thing for a while, finally seem ripped near the zipper. But seriously though I used to put my groceries in there when I would go to meijer... Check out the BugOut Gear sometime. Love those bags.
The Quicksilver backpack is smaller and I can still put my visor, rain gear, camera and it will only be half full. Has a cooler pack for refreshments built into the bag
Anyways, it is about as tall as a laptop (I put it in there sometimes) and can fit several full size text books in there. Just to give you an idea of scale.
I also did something most people haven't done. I put my radar detector in a camera pouch and velkro'd it to my shoulder strap. So the radar detector sits on my shoulder facing forward and can detect from the rear too (Valentine One radar). I hooked up a cigarette lighter cable that is simply an adapter to the battery tender and ran it up into my bag. I can plug in anything that uses a cigarette lighter plug, radar detector, cell phone charger, usb, etc. and then from that I have one wire running from the power plug to the detector, and another to the concealed display set up below my dash. So I see it below my speedo, but I hear it on my shoulder. Plus I never forget it when I get off the bike because the cable just disconnects when I walk away. So no one can steal it.
In the photo you'll see the wire running next to my foot (cigarette lighter/ battery tender cable adapter) runs into the bag where I have the cigarette lighter power cable plugged in. On it are two phone cable plugs, and one goes to the radar detector (bag on my shoulder) and the other goes underneath my arm and into the concealed display (black square underneath my speedo) This is a sloppy hook up on that day. Normally the wires are tucked into the bag and the radar is on the other shoulder now facing more parallel. Bag in the photo is the Quicksilver bag (Is a lot bigger when it's full)