hi jyoti,

you can set  paper size by right clicking outside of the report and selecting Report Properties, then got page there you can so many option select your desired option for example select A4 size the see the size details below text box width and height copy that in notepad or some where  then click ok,

now right side below you can see one property tab from there yo can see page size expand that and give whatever you copied width and height over here

You can get page setting by Right click on the blank area of the report -> Report Properties  -> Page Setup

 

There you can find to set the page size