Hi,
On CQ5.6.1 you can configure this in the OSGI console on the "Adobe Granite Workflow Service". Change the property "Max Inbox Query Size".
That being said, I would suggest purging some of your workflows - you'll get better performance when you keep the number of workflows low. There is a purge tool you can use to help with this [0]
Will
[0] http://labs.sixdimensions.com/blog/2013-10-03/new-aem-561-workflow-purge-scheduler/