SZ is an all-emcompassing (Height, breadth, mass, etc.) measure of the amount of volume taken up by a model. As the model in question is the Mounted Hussar, it would seem to me that the overall SZ is that of the largest model of a unit that includes a rider. Afterall, we do not add the SZ's of the models riding in a vehicle (Skimmer, transport, Purple Shark). Further, when a Praetorian Behemoth clobbers a model and picks it up from the ground, the Behemoth is not suddenly SZ 8 until he launches said unfortunate model.
Do not overthink some rules - they are what they are, and are, usually, ruled by common sense and black and white print (yes, I know that sometimes the two conflict).