Configurable, defaults to 50 000 bytes for now. Consider how to inform the user if the limit is exceeded.
I tested it by copying a long Wikipedia article in the field :D
I got two different warnings: "There are problems with some of your input." before the input field and the length error message below the field.
It would be great to have the first message to say "There are problems with some of your input. Please check the error message below."
Also we should follow the OOUI guidelines for errors https://phabricator.wikimedia.org/M241
That message is currently hard-coded in mediawiki core: https://github.com/wikimedia/mediawiki/blob/5612194879bd4f5c7d3e7c75d42b08bf91b84428/includes/htmlform/HTMLForm.php#L612
We are currently using the form error message from mediawiki core. But we could add our own styles on this page to adjust their styles. 🤔
PS: also compare the current design in Wikibase https://test.wikidata.org/wiki/Special:SetLabelDescriptionAliases/Q123/