Challenge: longest UTF-8 with valid domain name

Here is a little challenge that might end up in Simon's test suite
and be otherwise valuable for testing:

What is the longest (in terms of bytes) internationalized
domain name (when encoded as UTF-8)? Obviously because there
are characters that are ignored by nameprep, we would have to
ask for the longest one after ToUnicode.

The problem applies both to single labels as well as to
FQDNs.

Regards,    Martin.