Interestingly enough, based on my own experience- the maps themselves seem to be created as one - or at least created based on the zones around them - as the fit is almost exact. So it seems that they simply scale it to the size it needs to be to fit the WorldMapFrame, and then add the text. Which, really, makes sense.
I did a project a bit ago, that actually pieced together the individual map zones into their respective continents, using data from the Astrolabe library to correctly scale, and position the maps.
For the full size- well, fullest that DeviantArt would allow (~9200px wide), you can see that here
http://digitalutopia.deviantart.com/