Replacement Herman Miller Aeron Office Desk Chair Parts. OEM Replacement Herman Miller Aeron Office Desk Chair Polished Aluminum Base with Plastic Cylinder Gasket included. See picture of what is included for this listing. Aeron Fit / Size Chart. 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. We accept most major credit cards in our showroom. The Original AERON® Chair. Herman Miller Mirra® Chair. Herman Miller Embody® Chair. Herman Miller Sayl Chair®. Herman Miller Setu Chair®.