Here are the twenty most common surnames for italian families living in Italy:

Pos.	Surname
1	Rossi
2	Russo
3	Ferrari
4	Esposito
5	Bianchi
6	Romano
7	Colombo
8	Ricci
9	Marino
10	Greco
11	Bruno
12	Gallo
13	Conti
14	De Luca
15	Mancini
16	Costa
17	Giordano
18	Rizzo
19	Lombardi
20	Moretti

Even if Rossi is the most common last name in Italy (over 68'000 families), the name Mario Rossi (the italian equivalent of John Doe) isn't the most common name; here you find the 7 most common names and surnames:

Pos.	Name
1	Giuseppe Russo
2	Antonio Russo
3	Antonio Esposito
4	Giuseppe Rossi
5	Salvatore Russo
6	Francesco Russo
7	Giuseppe Esposito

Longest surnames (Source: Italian White Pages)

18 letters
Tschurtschenthaler
17 letters
Pasquadibisceglie, Di Francescantonio
15 letters
Monterubbianesi, Mastrofrancesco, Mastroberardino
14 letters
Abbracciavento, Castrogiovanni, Canavacciuolo, Colafranceschi

Shortest surnames (Source: Italian White Pages)

Bo, Pe, Po, Re, Wu, My, Mo, Hu.