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.
The smaller 8.5 mm and 11 mm are not very popular sizes. Hot glue fills the extra space well however for secure contact still. 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 to most common 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.