The initial size of segment will be
A segment is a subset of your audience that you create to run ads against. Segments are defined by audience conditions and filters, which allow you to target specific users who match the criteria for the segment you’re creating.
When you create a segment, it doesn’t start with any users. The initial size of the segment will be zero. Once people start meeting the targeting criteria, they’ll become included in the segment and the size of the segment will increase.