I have a 2 digit design - though I don't see it fitting in to a light bulb!
If I want ed to make something really small then I'd look at the people making watches and use their electronics.
My 2 tube clock is better suited to Z566/IN-18 sized tubes. In fact I have used the same hardware to drive four smaller tubes such as IN-8.
See details of the Dobly 3.1 clock here:
http://www.nixology.uk/dobly-3-1/(so called as it can drive 4 tubes but the 4th one can only display two of the cathodes so it's a 12hr clock with 0+1 or a 24hr clock with 1+2 (no leading 0!)
WIth 2 tubes - its a 24hr clock that shows time, date, temperature and air pressure.