It depends. If you want to do it at home, and you don't have a large format printer or a plotter, you can always print them in standard paper sized and then tile them together by gluing or taping them side by side.

If you're willing to pay for it, you can always take an image file to a place like Kinko's and they will print it out it out in large format for you. Kinko's had a limit on how large they will print, so you want an exceptionally large sized printout then you will need to visit a professional print shop. You can find those in your local yellow pages.