I read in the news about an Israeli designer, Yaakov Merdinger, who created the world's largest mezuzah case.
As you can see in the photo, the mezuzah case is bigger than the door.
Is this kosher?
A mezuzah case is really only a way to extend the halachik doorpost such that there is no need to actually insert the klaf into the doorpost itself, so inherently, size should not be a halachik issue. Even with regard to the mezuzah klaf (parchment) itself, as long as it can fit in the appropriate place on the doorpost (bottom of top third, not within the uppermost tefach *), there shouldn't be a problem. According to Ashkenazic custom you might also need a rather wide doorpost as well in order to be able to angle it. On the other hand, you would not need as tall a doorpost since the Ashkenazic custom is to place it at shoulder height if the doorway is very tall (see Shach 289:4 based on Yerushalmi; in contrast, see also Yalkut Yosef YD 285:77 who does not rule differently for tall doorways). See: http://www.halachipedia.com/index.php?title=Mezuzah:
The Kitzur Shulchan Aruch in סימן יא - הלכות מזוזה says that a Mezuza must be placed in the upper 3rd of the doorpost, but at least a Tefach (about 8 cm, 3") away from the lintel.
סעיף ה': מְקוֹמָהּ מִתְּחִלַּת שְׁלִישׁ הָעֶלְיוֹן שֶׁל גֹּבַהּ הַשָּׁעַר. קְבָעָהּ לְמַעְלָה מִזֶּה, כְּשֵׁרָה, וְהוּא שֶׁיַּרְחִיקֶנָּה מִן הַמַּשְׁקוֹף טֶפַח. קְבָעָהּ לְמַטָּה מִשְּׁלִישׁ הָעֶלְיוֹן, צָרִיךְ לַהֲסִירָהּ וּלְקָבְעָהּ כָּרָאוּי וּבִבְרָכָה. וְאִם קְבָעָהּ בְּטֶפַח הָעֶלְיוֹן, יְסִירָה גַּם כֵּן וְיִקְבְּעֶהָ כָּרָאוּי, אֲבָל לֹא יְבָרֵךְ.
So if the doorway is high enough, a tall Mezuza can be used. In this case, the doorpost would have to be about 5.70m (over 18 feet) tall. Such doors do exist, in Cape Canaveral.
However, the article is misleading, as it's not referring to a Mezuza, but a Mezuza-case. How to write a Kosher Mezuza that would be that tall, is unclear to me - unless the Sofer uses very, very large margins on top and below the writing. (A Mezuza is traditionally written on 22 lines; so (without monster margins) you're looking at letters about 4cm - 1.5" tall, since the space between the lines should be the same height as the letters. Try using a 108-point font to see what I'm referring to, then try find a quill or feather large enough to write that.)