Blockposters is a free service which allows you to create wall posters of any size from any size images! All you need to do then is, print individual blocks in your printer and stick them up together on your wall.

Just upload an image from your computer and choose how many sheets wide
you would like your poster to be once printed. Once you’re happy with your selection, you can download the PDF file containing your newly created images and print each one massively blown up, resulting in a
huge pixel poster to stick on your wall …

Try it : Blockposters.com