Just spent a week away from home sleeping in the car! First thing I did after purchase was remove the rear seats. Posted a thread with pictures but the search function is really bad - couldn't find it. Replaced the rear seats and the load area with a carpeted plywood sheet, hinged to access the battery and extra storage space.
I'm 5' 10" and with front seat all the way forward there is just enough room to lie fully stretched out with my 6' mattress. Bit more room diagonal even with my bicycle in the car (upright). But I'm usually sleeping on my side with knees bent. If you can sleep like that comfortably, you will do OK.
My prior car a PT Cruiser had fold down front passenger seat that created even more room. My career involved three month gigs away from home so I bought the car specifically for that. Plastic storage boxes in the rear matched the fold down front seat height creating an eight foot stretch to put my mattress on. I've considered finding someone who could modify the front Niro seat, but I'm not working much anymore (semi-retired) so I haven't gotten around to it.
2018 Kia LX HEV Metal Stream with Advanced Tech