This is all going very off-topic, but I understand the main problem is
ending up with edges on the I2C bus that have an RC time constant that
is too slow, so one needs to drop the pull-up resistance, and eventually
it wont be possible to drive the bus without some sort of line driver.
NXP have published an application note on how to run I2C along a (long)
transmission line, such as CAT5 cable, using a line driver:
The narrower I can make the track, the higher the characteristic
impedance, and therefore the easier it will be to drive (perhaps
allowing a line driver to be avoided). But that's no good if it can't
be reliably manufactured. Failing authoritative information, I would
probably go with 0.4 mm, similar to what Alan suggested, and a prayer to
the PCB goddess. I would prefer not to have to rely on divine
intervention, so I can blame something more substantive if it doesn't
work, but all I've found so far relates to current capacity, not
manufacturability.
Regards,
Robert
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