I don't believe there's anything larger than the MASSIVE Rodan CD-47 Nixie Tubes from Okaya Electric Co. Japan.
The tubes themselves stand over eight inches tall, with digits approximately 6 inches tall.   They'll light a room nicely.

The NixZilla clock is based entirely on the GPSII clock electronics package. The GPSII  HV power supply was modified 
to deliver a whopping 150 ma at 180vdc using a SEPIC circuit. That is a lot of juice. Those CD-47's consume 25ma per tube.
 

The six tubes are arranged parallel on a section of extrusion stock,with the clock electronics hidden inside. Colon posts are clear acrylic.
In all, three clocks were assembled. Two were presented as gifts. My personal clock has been running on a shelf in my office ever since.

I will be taking some images of the clock shortly. The only image onfile is the original prototype spread across a desk back in late 2002.

And for you tossers: the name "NixZilla" has been in use since 2002 to describe this giant clock.
Recently, I've seen no less than three individuals using NixZilla to describe their homebew clock.

 
 
 These Rodan CD47 / GR-414 tubes are extremely scarce, and highly sought after by collectors.

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