By default, printers usually won't print to the edge of the page so I have to add a margin.

You can check your printer's settings and see if you can set it to borderless. On Windows, you can find your printer settings through the Control Panel. On Mac, in System Preferences. When printing, make sure your web-browser doesn't add a margin back in.

If you've set your printer to borderless, then in Rapid Resizer's Options (in the step where you enter the size) you can set Margin to 0.

Whether you print with a margin or not, it doesn't affect the sizing of your printed design.