Once you have uploaded an image to the online script you may crop it and select the output size of the rasterized image by simply moving a slider into the exact position you want it to be. An information will tell you how many sheets of paper you need for the size that you have selected. Once you are satisfied with the size you may further customize the results by adding a border to it, selecting the dot size and selecting whether you would like a black and white image, a custom color image or a multi color image.

Once that is done you are set to go. The image will be processed and a pdf file will be created. Make sure you follow the directions of the developer when you print the pdf:

When printing the image with Adobe Reader, choose Page scaling: Fit to Printer Margins at the options window that comes up (Fit to Paper in older versions). If you chose horizontal paper alignment, also make sure Auto-Rotate and Center is selected. Most printers cannot print to the margins of the paper – these settings ensure that all the images will be completely printed. 

Thanks to Hans for this nice tip.