Whilst you could obviously just create lots of PNGs, or download them from a tile server, and then GIMP or photoshop them together, if you want to produce an A0 map at a print shop, you really want to get the map rendered as PDF, to ensure you get the full resolution of their printer. I’m not sure if you can force the PDF export, on the main page, to use a small scale and large area, at the same time.
There are also online tools that will render a street Atlas book as PDF. I’m not sure if that can be configured to generate a single sheet map.
Obviously, if you install the rendering tools yourself, you can create in any size within the capacity of the rendering machine.