I do this with high quality papers for the printer and even photopapers that shine and make a very cool poster. But all this work just make little posters from A4 or letter size of paper.
I tried many times to work with images to make them bigger and then put them togheter like a puzzle to make a more decent wall poster. This, of course, is not very easy and demand a little work with image editors that not every people will do.
Recently i found a site that makes exactly that, for free and in three simple steps.
Step 1 - You upload the image you like to convert
Step 2 - Slice the image
Step 3 - Download the final image ready forprint
It cant be easyer.