On Wikidata there are limits to the maximum allowed number of characters for "string" and "monolingual text" data types. On wikibase instances (different from Wikidata) it would be useful to be able to change and/or eliminate such limits, to adapt to the relevant needs.
Currently it is not possible to create monolingual string datavalues longer than 400 chars.
This is currently not configurable. It seems to be set by ValidatorBuilders.php#298 which just uses the default length which is hardcoded to 400 (ValidatorBuilders.php#181)
Configuration option to define arbitrary length monolingual string values for snaks
Given that the user tries to make a string value shorter than (or equal to) the defined max length the edit is successful
Given that the user tries to make a longer value than the defined max length the edit fails with a validation error.