Star caps are generally used with size 20 sockets & studs but can be used with size 16/14 sockets & studs if desired.
Star caps are slightly wider than sz20 round caps. If installing star caps with pliers, you will need a star cap die ($1). If installing with a snap press, you might be able to squeeze the star cap into the sz20 cap die, but otherwise you will need a star cap die for your press. Note that star cap dies for presses can also be used to install sz20 round caps but not vice versa.