With T188798, the length of the edit summary was set to 500 characters for all Wikimedia wikis.
However, when using the button "Änderungen verwerfen" offered by Flagged Revisions (ID mw-fr-submit-reject), the edit summary is still limited to 200 characters, causing the well-known issues e.g. when reverting edits by anon users with IPv6 addresses, cf. this discussion on dewiki.
The length of this text field should also be set to 500:
<input name="wpReason" size="120" value="..." id="wpReason" maxlength="500" class="webfonts-changed">