NEW Replacement Herman Miller Aeron Classic Seat Pan – Parts only – No Chair. New OEM Herman Miller Aeron Classic Seat Pan Replacement Size B Aeron Classic 3D03 Nickel. Herman Miller Aeron Chair OEM Replacement Seat Frame Pan Size B. Aeron Classic 3D03 Nickel. Classic Carbon – NO CHAIR IS INCLUDED. How can I determine my chair size if the label is gone? – To determine the size of your chair, feel underneath the top back lip of the chair’s backrest for a number of bumps. 1 bump is A, 2 bumps are B, and 3 bumps are C. One can also measure the seat pan width: Small A is 19 inches wide, Medium B is 20.25 inches wide and Large C is 21.5 inches wide. ONLY FOR THE CLASSIC STYLE AERON – Not the Remastered Version.