Different sizes of mezuzahs are sold by Judaica stores, the main sizes are 10cm, 12 cm and 15 cm. The larger the size of the mezuzah, the easier it is to write on it as the size of the letters is more.
People usually select either the 10 or 12 cm mezuzah for around the house, and the larger 15 cm Mezuzah for the main entrance doorpost to the house.