The example and code below show what you need to get auto items per page functionality for the pagination of a datagrid.

Resize the page vertically to see the number of rows being displayed per page adapt to the available height.

The AutoItemsPerPage parameter must be set to true and obviously Pagination must be used as well for this to work. Also, the DataGrid container must have styling that makes it automatially adapt to the available height.

An example of how that can be done for this demo site layout is shown in the <style> section below