Nickel free.

Small: Cap 1/4" (6 mm), Post 1/4" (6 mm), Base 1/4" (6 mm)
Medium: Cap 5/16" (8 mm), Post 5/16" (8 mm), Base 5/16" (8 mm)
Large: Cap 5/16" (8 mm), Post 1/2" (13 mm), Base 5/16" (8 mm)