Hi Pyb,
unfortunately this report is not very useful because it does not describe the problem well. If you have time and can still reproduce the problem, please read https://www.mediawiki.org/wiki/How_to_report_a_bug and add a more useful description to this report by providing exact steps to reproduce, which "search box" you refer to, preferably a testcase.
Also do you refer to the visual size of the search box? Or do you refer to how many characters can be entered?

Change 115197 had a related patch set uploaded by Tpt:
Increases size of the Special:IndexPages input

https://gerrit.wikimedia.org/r/115197

Change 115197 merged by jenkins-bot:
Increases size of the Special:IndexPages input

https://gerrit.wikimedia.org/r/115197

I've just increased the size of the search box. Should be deployed on Wikisource next Tuesday.