This is one of the aspect that many of us actually look for during the implementation- printing Navision reports on cutom paper sizes.
The answer to the same is simple, in the report designer donot put any papersize leave the same as blank. Run the report and previow the same. Now select the print option and set the paper size in which you want to print the report from the printer dialog box or you can use the paper sixe option.
Hope this will solve your problem.