Thanks for using Syncfusion products.
Query: “With "pageSize: 15" I want to display 15 groups.”
From your query, we understand that you want to display the grouped values based on the page size count. But as per the current architecture of EJ1 Grid it is not possible to achieve your requirement. We have processed the data based on the current page size (12 by default) records from dataSource not based on grouped record count. So it is not possible to define the grouped row count with the page size count in EJ1 Grid.