Sanjay:
This frustrated me awhile back, I was developing on two different machines; the report design page size was fine on one machine and not the other.
I was startled by the simplicity of the solution. The CR design view maximum size is determined by the maximum size paper your default printer can handle. My solution was to download a large format printer driver and install it on my development machine. I am then able to use virtually any size paper which once selected is reflected in my design mode screen.
Since I only develop to a size the user can print, what I have not tested is whether the user who only is viewing of the report is limited by the size of the paper their default printer can utiliize. Perhaps someone else can answer that point, but you should be able to do a quick test to see for yourself.
Hope this helps.
Regards,
John W.