All of our mezuzah scrolls are written on quality shelil parchment and certified kosher after being scanned by a computer (to check for extra or missing letters) and proofread for less discernible flaws by a professional mezuzah checker (magihah).
The first question is the size. Most mezuzah cases fit a mezuzah scroll that measures 6 cm, 7 cm, 10 cm, 12 cm or 15 cm in height. The two smallest sizes should be avoided; it’s very difficult for a sofer to write a kosher parchment in such small letters, and even if it does come out kosher, the writing is likely very low caliber, compared to what you can get for the same price on a medium-size mezuzah (i.e. 10-cm or 12-cm parchment). In recent years, particularly in religious areas, the 15-cm mezuzah has become increasingly popular. You may also come across 20-cm mezuzahs, which are primarily for Lubavitch followers and synagogues.
The look of your mezuzah case depends on your own aesthetic proclivities, yet there is one important detail to keep in mind: Is the doorway exposed to the elements? If the doorway is exposed to moisture, you'll definitely need a mezuzah case with a good seal. Many mezuzah cases have an inadequate seal, and some have none at all, just a big opening on the back, with a piece of tape covering the scroll to keep it in place. This means if you’re buying mezuzahs for interior doors (e.g. you live in an apartment building) anything works, but for the front and back doors of a house, you’ll want to check how the mezuzah is constructed.
Lucite or plastic mezuzah cases are often quite effective. A ceramic mezuzah may also be a good option. You can find very inexpensive plastic mezuzah cases for just a few dollars, as well as attractive decorative cases that are still affordable. Often well-designed plastic mezuzah cases are designed so that the scroll is inserted from the underside, with a silicon closure or a threaded bottom. Also, check the top to ensure that it is properly sealed.
If your doorpost is exposed to direct sunlight and high temperatures, choose plastic or wood over metal cases.
Also, transparent and translucent cases should be avoided in harsh sunlight, which can ruin the scroll, and bedroom doorposts, because the scroll should not be visible where people sometimes undress.
Beware: Many spectacular mezuzah holders come with a small "parchment" that has the words that go inside the mezuzah printed on regular paper, rather than written by hand on leather parchment by a trained scribe. Note that the piece of paper probably has the name of God written on it in Hebrew, so it should not be disposed of improperly, but. Bring it to a local rabbi. He will know what to do with it.
What exactly is a mezuzah? In common usage, the term "mezuzah" refers to the the parchment inside and the case which is visible to all – or both. Some people invest a lot of time and money trying to find a fabulous mezuzah case, when in fact the essence is the parchment inside. Ironically, today the mezuzah case may cost more than the parchment — and sometimes a lot more.
Are we fulfilling a mitzvah in any way when we buy a mezuzah case? We are enjoined to fulfill mitzvahs in an aesthetic manner: wear an attractive tallit, place a beautiful cover (me’il) on a Sefer Torah, decorate our sukkah, blow an attractive shofar and ensure that our tzitzit are in good condition. Likewise, embellishing the mezuzah with an attractive case is part of the proper fulfillment of the mitzvah.
The mitzvah of having a mezuzah on your doorposts seems very simple and straightforward, but a myriad of quetions can arise. Some are quite unlikely, such as would a Jewish president need mezuzahs in the White House, but many questions that may sound a bit outlandish can actually be fairly common.
Does a mezuza have to contain a scroll written by hand, or is a printed scroll also permitted?
A printed mezuzah scroll is hardly worth the paper it’s written on! A mezuzah definitely must be written on specially prepared leather by a qualified scribe.
Is a mezuzah effective as an amulet, e.g. in a car or a pocket?
No. The mitzvah is to place a mezuza on the doorposts of your home. However, the Talmudic Sages noted that a mezuzah differs from other mitzvas in its power to protect: Typically a mitzvah provides protection while a person is actively engaged in it, whereas the mezuzah involves a one-time action that provides ongoing protection.
What should you do when you pass by a mezuza?
According to one opinion, when setting out on an out-of-town journey one should place a hand on the mezuzah and say, “Adonoi yishmor tzeisi uvoi” (“May G-d guard [me upon] my departure and return”).
The prevalent custom is to place your fingers on the mezuza and kiss them. Others merely look at or touch it and contemplate the words written inside.
What effect does a mezuzah have?
“Be scientific: try it.” Post a kosher mezuza and contemplate it regularly.
The Talmudic Sages said, “He who has tefillin on his head and arm, tzitzit on his garment and a mezuza on his doorway is certain not to sin,” since he has constant reminders.
In a rented house or apartment, who has to buy and post mezuzas — the tenant or the landlord?
The obligation to post a mezuzah is incumbent on the tenant, regardless of whether the landlord is Jewish or not.
How long are you allotted to post a mezuza?
If you rent a home outside the Land of Israel or accommodations inside the Land of Israel, you are exempt for thirty days. If you rent a home inside the Land of Israel, you must post a mezuza immediately.
How often must a mezuza be checked?
Twice every seven years, or whenever a concern arises that the parchment may have sustained water damage or was stolen.
Which doorways require a mezuza?
All doorways of the home. For some people just getting started with the mitzvah, the monetary outlay may be daunting. What is the source that all rooms require a mezuzah? The Rambam writes, "When there is a separate room in a house, or even one room which leads to another room, it is necessary to affix a mezuzah on the doorway to the innermost room, the doorway to the outer room, and the doorway to the house, since all of them are used for the purpose of dwelling and are permanent structures."
The Torah does not say uch'savtam al mezuzas beisecha, write them the doorpost of your house, but rather uch'savtam al mezuzos beisecha, write them on the doorposts of your house, i.e. there should be a mezuzah on every doorpost in one's home, not only the entrance.
Synagogues are exempt from the mitzvah of mezuzah because they are not dwelling places, however, the custom is to affix a mezuzah at the entrance to a syngagogue.
A doorway to an unclean place (e.g. a bathroom) should not have a mezuzas. Similarly a very small room (less than four cubits by four cubits, approximately 2 meters x 2 meters) does not require a mezuza since it is considered too little to be used as a proper dwelling space. However, a small room that leads to a large room does require a mezuza.
Does a temporary dwelling need a mezuza?
No. Therefore a succah is exempt from the mitzva of mezuza.