You will need to write a plugin with a filter module. Here are the plugin tutorial and the filter module docs:
http://docs.question2answer.org/plugins/tutorial/
http://docs.question2answer.org/plugins/modules-filter
Specifically, take a look at the filter_question method.
You can use this plugin as a reference: https://github.com/pupi1985/q2a-maximum-tag-length
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