Wheee~ And err, this website~
http://www.blockposters.com/
I chose one A4 page width for all of them, because if you choose more you have to print out multiple and stick them together to get a huge poster. Also, the image can't be too large (the one I'mma give you is the right size & format), and has to be in .jpg/.jpeg form. Anyway, here's the Lyra poster I have, if you want to use it;