A hard-sided bag will be a very snug fit through the trunk opening. My guess, based on measurements I did for the fitted luggage, is that the smaller ones will fit, but the bigger ones that are at the max of what airlines allow for carry-ons will be touch & go.
It will however, fit in the passenger footwell with no problem, and the passenger's legs can stretch out over the top of it.
The trunk is only 10" deep.
For a frame of reference, my Tumi roller is 10" deep, 14" wide and 24" tall, including the handle.
Heck, my helmet is 10" wide...