have a look at the various manufacturers websites, you can get loads of different sized cases. i have 3x 44ltr kappa cases,
i keep the topbox empty to put my helmet and jacket in when i stop,
a large 3man tent and 3season bag goes in one side (im a big fat bastard who likes his space, you can get much smaller tents!)
and that leaves 44ltrs for bits and bobs, you will be amazed at what you can take, i took far too much junk with me the 1st time i went anywhere.
"whatever luggage you have, you will try to fill" thats very true, as soon as i thought about what i really wanted i halved the junk, and now i have room for my giant tent and air matress, AND have a spare box for helmet/jacket or

!
i know you are limited to what you can carry size wise on a CG, but you can get really tiny high performance tents and sleeping bags these days, that pack up to little bundles. they can cost a bit though. and you already know you can go as wide as a skateboard!
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dave