Two passages of Torah are written in the mezuzah: 'Shema' [Deut. 6: 4-9] and 'Vehaya im shamo'a' [Deut 11:13-21]. They are written in in one column on one sheet. They are included as they are the two sections that make reference to the specific commandment of mezuzah:

and you shall write them...

(and you shall write them on the doorposts of your home and your gates). Originally the word mezuzah referred to the doorpost itself not the parchment and indeed mezuzot parchments could also be stored in hollowed out parts of the doorpost (wood or stone) not affixed in a container. The writing must be done in kosher black ink and must be on lines ruled with a sargel. Even though the scroll is hidden from view it must be beautiful writing and straight. (Tefillin are exempt from ruled lines other than the top one - though no sofer would do it without). It must be written in k'tav ashurit (assyrian script) - nothing else is permitted. According to Sofrim 9:4 originally all the letters in Shema Yisrael (Hear O Israel) were enlarged, not just the ayin. Sforno explains that the ayin is enlarged as it means the word means eye with which we percieve/witness God's Oneness.