Once you've decided which size goes best for the needs and the future, just let us (dba) know. Obviously the smaller the better, but that is probably not possible :-)

I had to modify it on an installation. I modified in LocalSettings.php the parameter $wgWBRepoSettings['string-limits']['multilang']['length'] (default is 250) and augmented accordingly the two columns term_text and term_search_key of the table wb_terms (default is 255). Is there anything else to change?

Also it is worth noting that the unit in MySQL is the byte for strings encoded in UTF-8, at the contrary of the number in LocalSetings.php which is the character, hence it would be better to fix the maximum length in MySQL to 4 times the number in LS.php else the strings are truncated in MySQL and the uniqueness constraint provided by these MySQL columns could be weakened for non-ASCII characters.

@Seb35 What you describe here sounds like T142691: [Bug] wb_terms table truncates labels exceeding 255 bytes, possibly leaving invalid UTF-8. This task is about the columns storing entity ids as string (like Q123 or P456).

(While change_object_id belongs to a WikibaseRepo table, it’s still part of the change dispatching system, so we expect that this whole task belongs with the Wikidata Integrations Team.)