Mitzvah to hang the mezuzah on the doorpost is one of the 613 mitzvot given in Torah to Jewish people.
Mezuzot are completely amazing creatures – I can’t call them things as they seem to have their own personality and will.
Mezuzah is an energy creature with two-way connection to the area (room or house) that it is protecting –
it infuses the area with energy of holiness and it takes a hit from the negativity that is happening in that room (house). This is a primary reason Mezuzah can become posul (invalid – if even one letter breaks on the parchment it’s written on). If Mezuzah is posul, it not only does not protect, but it projects into reality the very defect that is imprinted into it. That’s why the Halacha is to check mezuzot at least twice in 7 years – some people do it every year.
There is a well known in Rav in Yerushalaim (Rav Yom Tov) called “Mezuzah-Man” – his interesting ability is to tell people their issues by looking at their mezuzot – because when mezuzah hangs in a house long enough, it takes an imprint of everything that is happening inside (The breaking down is an extreme case when it can’t “hold it” anymore.)
Now interesting discovery I made recently – if the mezuzah is kosher and you channel energy into it (I read Tehillim with my hands on mezuzah and feel the energy flow into the mezuzah), it has tremendous positive impact on the room (and everyone inside) – much more than if you would read Tehillim or channel energy into the people directly. It works like a spiritual amplifier and a battery, so if you “recharge” it daily (or once in a few days – see how long does it “hold the charge” in your case), you can create really powerful healing environment in your house or the room where you live or sleep (I usually do it when kids are asleep – they are most open at the time).
Feeling the energy isn’t a prerequisite however. It’s enough if you focus your attention on mezuzah while reading the Tehillim, or simply focus on it with thoughts of love and affection.
The custom to kiss the mezuzah as we pass through the door accomplishes exactly that – if done properly. In fact this is how I made my discovery – when I asked a question why do we do it? Unfortunately many people barely notice the mezuzah when they enter – we need to give it more of our love and awareness!
Just to mention that in order to work, the mezuzah must be kosher (e.g. written on the parchment by qualified Sofer observing all the Halachic specifications) and bought from a reliable source.
Putting a zerox copy on the doorpost is like taking a picture of the radio-receiver and expecting it to play music. If the radio-receiver is assembled not according to the spec, it simply doesn’t work, or worse, creates distorted sound.
The reason some people go for larger or more beautiful/special mezuzot is that not all kosher mezuzot are created equal. The energetic capacity of the mezuzah depends on the care with which the Sofer writes it, his purity of thought and generally what kind of a person he is. In modern manufacturing-style resale system it’s difficult to ascertain those factors, so most people look at the external things like size, style of the ktav (writing) and beauty of the ktav. Many good mezuzot come with certificate of quality, certifying those parameters. I usually like to go through a few mezuzot on sale, touching them gently and feeling them. I still smile as I recall my latest buy – the mezuzah just didn’t want to leave my hands once I picked it up 🙂
I however had seen good results with “recharging” procedure even with regular 12 cm mezuzot – they just need to be real kosher mezuzot.
Hope it helps!