Report Builder will print *exactly* what you have layed out on the page. It does not have an auto fit feature
If we take A4 paper for example, it's 21cm wide. If you have the default 2cm margins this leaves you with 17cm of printable space. If each of your 11 columns are 2cm wide this means 5cm of your report would need to be printed on a second page.
The best way to fit this to 1 page is to reduce the width of your columns and maybe reduce your font size until everything fits into the printable area. Reducing the margins can help too, but many printers have limits on how small the margins can go.