Hi, DR, not particularly large, 25-30l,,although a larger bag would be okay. I can barely fit a week's worth of gear into the side cases, would be nice to have extra room.
Quick attach/removal imperative.
In the 30 liter class one of my favorites is the BMW small tail bag (small in a misnomer).
The bag is basically a 30 liter bag with a waterproof inner membrane (use the inner or not as you wish)
It is expandable to about 35 liters if needed (I have seldom needed to).
It looks pretty decent on a BMW bike (mainly made for the BMW GS but it comes with lots of strapping extras to fit about anything).
One nice feature is IF your rear luggage rack/plate has a small rear lip behind the support stands there is a pocket on the bottom of the bag to slide over that lip so you only need to secure the front with quick disconnect buckles then slips off & is easily carried as luggage.
Size is around 13" tall x 16" wide x 13" long. (when full it also becomes a passenger back rest)
Might be worth a look at your local BMW dealer (they usually offer this bag on sale about twice a year & when on sale not a bad bargain for it's usefulness)