I just purchased the large Tusk boxes. I still need to mount them (I have SW-Motech racks).
The decision between the medium and large came down to being able to fit my riding gear in it. The large is no problem even with the back armor/pad installed in the jacket. The medium box would be entirely consumed by the jacket alone.
I have a set of Hepco & Becker pannier frames for sale from my 2016 NC I recently sold. These are the quick release type. Bolt the mounting plates to the footpeg carriers and near the grab rails and it takes about 2 minutes max to fit or remove the pannier frames. Can be used with genuine Hepco & Becker boxes or soft luggage. I am in Australia. Exchange rate is very positive to buy from Australia at the moment. Very Good Condition at $250 AUD = $171 USD.