Resurrecting an old thread, but if you take a huge map of 128 hexes across, or 10240 hexes total, you can get an idea of the size of a map that encompasses the Earth.
The total surface area of the earth is 196,950,000 square miles, so dividing that by 10240 hexes is 19,233 square miles per hex. That's about the size of Costa Rica or West Virginia. Each side of a hex is 86 miles across, the perimeter would be 516 miles and the distance between parallel sides would be 148 miles. That also has the unfotunate side affect of making the equator 37,888 miles long and the meridian 27,520 meters long. Kind of a squashed earth, but we are talking about a flat projection divided into hexes here.
That also means a huge map contains roughly the following:
Ten and a half Duel maps (18,465,216 Sq Mi, 4,128 mi height x 5,920 mi width, the size of Asia)
Five Tiny maps (38,777,318 Sq Mi, 6,192 mi height x 8,288 mi width)
Three and a half Small maps (53,316,188 Sq Mi, 7,224 mi height x 9,768 mi width, the size of the Pacific ocean)
Two and a half Standard maps (80,028,443 Sq Mi, 8,944 mi height x 11,840 mi width)
One and a half Large maps (128,055,916 Sq Mi, 11,008 mi height x 15,392 mi width)
Each hex would remain 19,233 square miles each time.
By the way, with minimum separation for every city to receive its 37 tiles, you could fit about 276 cities on a huge map if it were all land. The Earth is mostly water though, so that would be 2,980 land hexes, or about 80 cities with all its territory on land. That should be plenty to fit all 206 sovereign states of the world on one map (too bad the game can only handle 63 nations and city-states).