Definition of Batch size
The batch size is the number of work items that are delivered as a single unit but are not necessarily executed at the same time.
Therefore, you express batch size as a number (the number of work items). By using small ‘batches,’ the work is relatively quicker to complete and its value can be demonstrated sooner.
Optimizing the batch size is a way to improve flow. Finding the right batch size requires regular inspection and adaptation by the Scrum Team. For example, by discussing Lead Time during the Sprint Retrospective.
It is advisable to record the batch size as a work agreement in the definition of Workflow.
Note: It is a misconception that batch size is the same as Sprint length. For instance, a Scrum Team could deliver and implement one PBI after another during their Sprint. In that case, their batch size is 1 and thus certainly not the same as the Sprint length!