Since being involved with designing many cars over the years.... I can tell you that all of the manufacturers design the cabin for a SAE 95th percentile occupant to meet the FMVSS / Federal Motor Vehicle Safety Standards. There are no other larger sizes that a manufacturer designs a car for in the driver's seat, they must design a vehicle based on a 95th percentile manikin to pass federal standards.
The dimensions of a 95th percentile occupant determines many vehicle dimensions, from distance to occupant collision to interior parts, bolsters, cant rails, dash, side door, roof inner, side glass, etc., then vision studies, forward through the windscreen, side mirrors, dash instruments, wiper zone, then seating position, the location of the hip point, ball of foot to the pedals, leg angles, hip angle, steering wheel, shifter, etc., then airbags, then weight of vehicle for testing, all of this is also based on the seat track adjustment variables and frontal, offset barrier, roof and side impact intrusion.
There can be additional head room clearance, etc., but basically all of the clearance dimensions are defined as a minimum clearance requirement regarding crash based on a 95th percentile manikin, yet, still having to meet all of the aforementioned requirements.
Here's an example from the net of the SAE 95th percentile occupant / manikin dimensions:
http://www.dentonatd.com/dentonatd/pdf/HIII95M.PDF
Cheers,
Mike