> I also wonder how much useful space is left in them after the slide-out steering mechanism, batteries and motors.

looking at the image in TFA, that looks about the size of a carry on which is much smaller than I was imagining. to counter, there's a Visa commercial running as part of the Olympics with Simone Biles next to her suitcase which is as big as she is. It is disturbingly large. Like, it can't be real, can it? Or she is just that little. Either way, I'm remembering the wrong parts of their commercial, but I could see a suitcase of that size being something Transformer-ish into something you and your family could ride