Mezuzah are the parchment scrolls which Jews attach to the right doorposts of their homes to fulfill what is written in the Torah as understood by our Sages:
"and you shall write them on the doorposts (Mezuzah) of your house and upon your gates."
The two sections of the Torah in which this verse appears, (the first two paragraphs of the Shema prayer), are written on parchment,
rolled up and attached to the upper third of the doorpost, usually enclosed inside a decorative container.
The Biblical texts must be written by hand, in order, on scored parchment.
How are Mezuzahs written?
Today Mezuzot are generally written in the standard sizes: 6 cm (about 2.5"), 10 cm (4"), 12 cm (about 5"), and 15 cm (6").
As you might imagine, it is quite difficult to write 21 lines of script in 2.5".
Thus, even though they are smaller, good little Mezuzah often are more expensive than the cheaper larger ones.
Where do you put up a Mezuzah ?
Every Jewish home - owning family should have a Mezuzah on their front door post, and every entrance to their home.
Many Jews also put Mezuzah on the entrance to every bed and / or living room in the home.
Other than in the home, you may also put a Mezuzah in your car.
Car Mezuzahs may be placed anywhere in the car, at any, or every entrance of your choice.