You'll want the image itself the same size as the poster so that the print people don't have to resize it. I think you could go quite a bit bigger than A3. Based on cursory research it looks like the original was B1, 67 x 97 cm.
It's probably easiest to find a place that will do custom full bleed posters, see what their standard sizes are, and work from there (making everything much cheaper/easier).
When I did mine the resized image was inevitably slightly off the standard sizes, so IIRC I added an inch or so of blank space on one end and then manually filled it using the clone stamp (probably a very low-tech way of doing it).