Each Bottle Filter has size marked on the Hollow Fibre Cartridge, which is the clear plastic filter with white membranes inside. The size is marked as "S", "M", or "L" at the top of the Filter.
Size Small fits the 20oz Tritan™ Renew bottle
Size Medium fits the 32oz Tritan™ Renew bottles
Size Large fits the 32oz Vacuum Insulated Stainless Steel bottles