On the page where you enter the size:

  1. Click on the big Print button. Then on Proceed.
  2. The PDF of your resized design will appear.
  3. Touch the share button (the rectangle with an upwards arrow) at the edge of the screen.
  4. Touch the Print button. You may have to scroll down to reveal the Print button.

This assumes you have an AirPrint supporting printer setup on your wireless network. (AirPrint is Apple's standard for connecting to printers. If you've been able to print other things from your iPhone/iPad, Rapid Resizer will be able to print.)