It's not possible to change the "rows per page" when using a report with runtime parameters.

To reproduce:

1) Create a report using runtime parameters. A simple example one might be:

select * from items where holdingbranch = <<holding|branches>> and homebranch = <<home|branches>>

2) Run the report and make sure you have more than 20 results.
3) Select a higher number for "rows per page" and click "Update"
4) Verify you are taken back and have to reenter your runtime parameters.
5) Verify on running the report agian, the rows per page is again 20.