Blockposter is a great site for uploading images so that you can make large posters. There's lots of options and it's very easy to use. There's no sign ups and it's free unless you want the more detailed options. It renders a PDF in your choice of size that you can download and print, putting it together like a jigsaw. You can get even more variation by printing in either A4 or A3. I printed and laminated a big world map that we've used in our classroom for years and having a visual reference for places in the world that's so easy to access has been a boon! It's also been great for those large spaces in classrooms or corridors that need big impact.
Just a quick note on maps. I always use the ones that most accurately portray the size of countries in relation to each other. Quick tip: If Greenland looks as big as Africa, find a better map!
Example of a blockposter PDF Blockposter too hard too easy 3