To fit a pair of jeans, change of underwear,  a toothbrush, and the other bare necessities for a weekend?

10L? 18L?

Thanks

i have a rear bag for my 696 it is shaped different than a tank bag but it is 18L and its still small in my opinion.
a light sweatshirt and long sleave t-shirt and light tenis shoes its almost maxed out.
10L is way to small.
Go Big or Go Home
my 2 cents

As with any other gear, I like to try out tank bags in person. Getting a bag that's too big can get in the way of handling. Too small and you won't be able to fit anything useful in it.

Oh, packing tip:
Try rolling up your clothes. It's amazing how much less space they take up when you roll them up. When I go home for the weekend I bring a pair of pants, 2 shirts, 2 pairs of underwear and socks, sleeping clothes and lay them out flat in my bag. On the trip home I roll up the used clothes so that I have more space for the goodies that I steal from my parents. 

My TourMaster is 12L, expands (front only) to about 15/16. http://www.tourmaster.com/xcart/product.php?productid=138&cat=32

With a pair of shoes and the things you mention, it will be full, but still fine.

If you have a plastic tank, you'll need a strap mount version. IMO, that's just enough hassle to choose a back pack unless we're talking about longer rides/touring. If you have a metal tank, it's much easier (magnet bag).

i can always get away with just having flip flops as footwear. Its for rides into N. Carolina for the weekend and the roads are a bit too twisty and long for me to want to be stuck to having a backpack with all my crap on.

So far the plastic gas tank is the only downside to my S4rt.

Setting up straps for a tank bag can be a drag but once you get used to it can be pretty quick. Just don't slack off and leave them on there when you're not carrying your bag. They scratch the hell out of your tank really quickly if there's no bag to hold them down.

http://www.nelsonrigg.com/pages/Sport-Touring-Products/CL450-Big.htm

I use this all the time, as I commute on my bike. This definitely fits the things you've listed... shoes, jeans, couple shirts and underwear, toiletries... I can fit that list without expanding the bag. I regularly get a pair of shoes, pants, half-folio (for work papers), and even a frozen dinner. The side pockets have spare gloves, summer gloves, gum and sunscreen. I could probably squeeze a little more into it.

I also like the curve at the back, as it makes tucking down easier. I imagine this would be even more noticeable if you have clipons.

Fuel ups involve opening the 2 front straps and flipping the bag onto the seat, leaving the back strap connected. Only takes a moment.

As for the straps... PITA if you took them on and off regularly. The front strap (one strap is provided for BOTH front clips) is threaded through the frame between the neck and airbox, and the rear circles the hinge at the back of the tank, under the seat. I ended up using a bungee net over the tank to hold the straps in place when the bag isn't there. In addition, I removed the foam pad that came on the bottom of the bag, so the bungee doubles for that.

Disclaimer... Over time, the buckles have scratched the tank. I have a Dark, and I like the "worn and abused" look, so this works for me. I can't imagine doing the same setup on a gloss tank.

Stopped by NPR Ducati on saturday, and ended up getting a ducati performance bag, it was more expensive than the Tourmaster for sure, but it expands to 15L and on the ride back worked perfectly to fit the other stuff I had bought while there.

Thanks again