If it's printing smaller than expected, you probably need to crop away the area around the edges of the image that aren't part of the pattern. Go to the page where you enter your design's size and click Prepare Your Design to Resize Accurately. That will take you through the steps to straighten and crop your design.
Also, on the page where you enter the size you want, ensure the Paper Size option below is correct. Don't use the "Fit: size of design" paper size option unless you're using a large format printer.
If your design is still printing the wrong size, when you're at the last step of printing in your browser, ensure no option is set to shrink, fit or scale your design. You want it to print at the actual (resized) size with 100% scale. For example, Actual size should be on and Fit to page off. In Chrome's PDF printer, you have to click More settings to reveal these.
Google's free Chrome web-browser is the recommended browser for accessing Rapid Resizer. Switching to it might help. On iPads & iPhones, Safari is the recommended browser.
The size you give is for the entire design. If you want to size in terms of a part, click on the Part button below where you would have entered the overall height. For example, if you wanted to specify the height of letters above their baseline, click on the Part button and draw a vertical line from the top to the bottom of one letter.
If you're trying to print to the exact full-size of one or more pages, that will take more pages because (unless your printer is setup for borderless printing) there has to be some blank space around the edge of each page.
If you want to print at the same size as the original that you scanned in, you'll have to measure it with a ruler and enter that into Rapid Resizer. Make sure to measure the same section that's been cropped to in Rapid Resizer.