Excellent quality ferrules for your ski pole tips. Use hot glue to install. To remove old ferrules or snow baskets use heat such as heat gun or boiling water to loosen glue. Size is the width of the end of the pole. Make sure you put them on pointing forward to dig straight down into the asphalt.
10 mm is most popular size. The smaller 8.5 mm and 11 mm are more uncommon sizes. Only use heat sensitive glue when installing such as hot glue. Heat up the end of the pole as well as the ferrule some before sliding on. Be sure ferrule slides on far enough to be solid connection to pole.
These ferrules add about 5 mm in height compared to most snow baskets to help lengthen your poles slightly for rollerskiing, which depending on the rollerski frame height set you higher off the ground than snow skis. This extra 5mm should make your regular skate poles long enough when skating on the Pursuit Fork Flex for example.