Turn the Aeron office chair around so you're looking at the seat back.
Place your fingertips under the frame edge near the top center of the seatback near the Herman Miller stamp. Press under the edge until you feel one or more bumps. Count the number of bumps with your fingertips to determine the chair size -- one bump is an "A" size chair, two bumps a "B" size chair and three bumps organized in a triangle is a "C" size chair.
Face the front of the chair and run a measuring tape from one side of the seat frame to the other to measure the seat as another method of determining the chair size: 19 inches wide is an "A" chair, 20.25 inches wide a "B" chair and 21.5 inches wide a "C" chair.