Step One: Get the high resolution version
First click on the thumbnail of the number card you want to print to see if a higher-resolution version has been posted. Most of the cards on the WE and AE 'forms' pages, for example, have high-res images already posted. If there is a higher res version, download the image by right clicking on the larger image and saving it to your hard drive.
Step Two Option 1): Print on a home printer from your photo software
You will have to experiment with your own software to print the right size. This is a bit of a trial and error process, and there are two ways to go about it. One is to set your printer 'setup' at a few different DPIs settings (dots per inch), try printing, and see which most closely approximates the right size. The other is to use photo software to downsize the image so that it prints perfectly on your particular printer.
Step Two Option 2) Print using an online service (like Kodak Gallery or Shutterfly) As with home printers, you may have to experiment with layout to get the sizing right. I have found Kodak Gallery's quality to be the best at reproducing these.