On the webpage where you enter your design's size, click the Fit to Page button.

There will still be a margin around the page because, by default, printers usually won't print to the edge of the page. If you need to print to the entire area of a page, you have to go into your printer's settings and set it to Borderless printing if your printer has that option. Then in the Rapid Resizer step where you enter the size you want, first click Options (near the bottom of the controls on the left) and set the Page Margin to 0.