Project Management Tools and Techniques | by Business Management Consultants

Subdividing the WBS
The idea is to divide the total work of the project into major groups and then to subdivide these groups into smaller and smaller parts until you reach units of a size adequate to provide control of the project. The lowest level of subdivided work should be small enough to permit adequate control and visibility without creating an unwieldy administrative burden. By that, we mean a level at which you can assign tasks and accurately estimate task scope, effort, resources, cost, and schedule.

The 80 hour rule, or 2-week rule
Some experts suggest breaking down WBS elements so that no work package exceeds 80 hours of effort or 2 weeks of duration. Then control milestones will not exceed 80 hours of work or 2 weeks of duration. This seems granular enough for awareness and control.

The sanity rule
Other experts suggest breaking WBS elements down only as far as you need to monitor progress regardless of the duration or resource requirements of the work packages.