I am not sure whether this suggestion suitable for you or not.
Go to Control Panel and Printers and Faxes. In the Printers and Faxes, go to menu File->Server Properties, then under the Forms tab, chose 1 of the forms and tick on the Create a new form, then modify the paper size, give the form name, and save it.
In the default printer, select this paper size to be used and it will reflect in the crystal report designer screen.