John C. J. M. Wright asks whether one takes only the first name of one’s Confirmation saint, or both first and last.
First off — what did the guy who confirmed you say, and what did you request as your name? “Justin” or “Justin Martyr”? That would be the usual determinant. (If the deacon/priest/bishop mispronounces your name, of course this doesn’t affect anything. You get the name you asked to have. If you had asked to be baptized with the name of the locally sainted Bishop Lucifer of Cagliari, and the priest had given you a steely-eyed stare and firmly baptized you “John”, your baptismal name would be John, and serve you right.)
There is nothing wrong with baptizing or confirming someone with a long saint’s name that includes the saint’s nickname or surname. The most common example in America are the countless Irish gentlemen named “Francis Xavier O’Someclan” after St. Francis Xavier. (Hence all the “F.X. O’Someclan” folks, too.) But there are plenty of people named “Juan de la Cruz Spanishfamilyname”, “Lawrence O’Toole McSomeclan”, and the ever popular “Charlemagne”. The French have traditionally been much given to hyphenated names of this sort, whether or not they use them these days. There was even some obscure fellow who was baptized Joannes Chrysostomus Wolfgangus Theophilus Mozart.
So feel perfectly free to style yourself “John Charles Justin-Martyr Wright”, if that is in fact your name. You will be following the conventions with perfect propriety.