the internal width of the car from one internal door panels to the opposite panel
With a ruler.
You could measure the size of a table or the size of a room, or the height of your mother or the length of a car.
The 2007 Toyota Camry has 57.8 in. of front shoulder room.
The 2005 Chevrolet Impala has 58.9 in. of rear shoulder room.
The 2006 Hyundai Elantra has 53.5 in. of rear shoulder room.
The 2013 Nissan Xterra has 58.3 in. of front shoulder room.
The 2011 Volkswagen GTI has 54.7 in. of front shoulder room.
The 2003 Volkswagen GTI has 52.7 in. of rear shoulder room.
The 2005 Kia Sorento has 58.4 in. of rear shoulder room.
The 2002 Nissan Sentra has 52.6 in. of rear shoulder room.
The 2007 Mitsubishi Outlander has 56.4 in. of front shoulder room.