its apply on all the pages. actually i have a header with 25px so if i change the paper size it will cut my header.

If you want to do like this , then you need to customize the report header. you need to replace the header of base module. you need to change by inheriting this ***>report.external_layout_header**, or if you make changes in existing report yo need to do like adding hight and width ****>

I already do that but problem is space for header repeat in every page.