In case anyone was curious, I just visited the world 5 map and stitched a section of screenshots of the map at full zoom from end to end and it ended up being exactly 25000 pixels wide.
Given that each pixel on the map is a 1m square tile, this comes to 25km per side.
This game is impressively large, especially considering how densely it's populated with visible structures.
You can compare the 241sq miles to other game maps here