What is Work Package?

A work package is the lowest level of the WBS — a chunk of work small enough to be reliably estimated for cost and duration, assigned to a single owner, and tracked. Everything above it in the WBS is grouping; the work package is where work actually gets planned and measured.

Practical sizing heuristic: if you can't estimate it or one person can't own it, decompose further; if you're tracking individual to-dos, you've gone too far.