Well how much stuff are you going to carry? Remember that when you are determining sissy bar bag that's too heavy or rides too high can make your motorcycle unwieldy or difficult to steer.
Also, the extra weight of a really heavy bag could lead to a broken sissy bar, luggage rack, or even part of your bike's frame. Once you figure out how much stuff you want to have with you while you ride then you will be in good shape to pick the right size bag. You might think about getting a set of bags rather than just one.