Giant sized mezuzos can be a good way to really get into the mitzvah of mezuzos because they are so easy to notice when you go into a room. They truly are impressive. When the ksav (writing) is top quality they are magnificent! They do however have some downsides. It is hard to find practical cases for them. When you can find cases, they are hard to roll tight enough to fit in the case without damaging them. The mezuzos themselves are very expensive, starting around $200 for something only mediocre whereas for $200 you could get a top of the line 15 cm mezuzah that would be far better than what you would get for that price in a 20 cm or larger size. The letters in a giant mezuzah tend to have too much ink which often lead to cracks in the letters. The cases are also more expensive and the selection is more limited. All in all, if you really want a huge mezuzah, go for it but don’t skimp on the price. Be ready to spend as much as five hundred or even six hundred dollars. Make sure if you are placing it outside that you properly waterproof it. It’s best to limit these super sized mezuzos to a few key locations and stick with a more practical 15 cm size for the rest of the house.
At 40 cm, the two cases on the bottom of the picture would be able to hold just about any mezuzah you can find. I know that if you have been in Jerusalem you may have seen mezuzos of a truly massive size but those would look out of place on all but the most massive doors. In addition, they would be available only by special order.
This case is my favorite from among all the large cases because it is weatherproof and very solid. I like solid things in general (think Starek tefillin battim) and mezuzah cases are no exception. This case can be ordered from me for $65.