The following resources are helpful for planning capacity of Nintex Workflow databases.
Many thanks to Cameron McConnell for his assistance in compiling this list.
Database Design Guide: Nintex Workflow
Nintex Workflow Backup and Restore
Disaster Recovery Planning Guide: Nintex Workflow
Splitting existing SharePoint and Nintex Content Databases
SharePoint limits related to Nintex Workflow
Purge Workflow Data - Reduce the size of the workflow progress database table
How to purge large history list and WorkflowProgress table
How to purge WorkflowLog table
Demystifying Workflow History (Part 1)
Optimizing timeout settings: SharePoint timer service
Collection variable performance considerations
Supplemental documentation:
NWAdmin Operations - Nintex Workflow 2013
NWAdmin Operations - Nintex Workflow 2010
