Just some numbers for Giant Earth Map (GEM) Scale :
Earth Surface : 510.000.000 km^2 (see Wikipedia)
Giant Earth Map : 180 x 90 = 16.200 Hex (Tiles)
=> 510.000.000 km^2 / 16.200 Hex = ca. 31.500 km^2 / Hex
31.500 km^2 is about the area of a circle with radius 100 km.
(area = PI x 100 km x 100 km)
So most or all Metropolitan Areas of the biggest cities on earth are smaller than one Hex,
e.g. Greater Tokyo Area : Pop 35,676,000, Area 13,500 km^2
e.g. New York Metropolitan Area : Pop 23,508,600, Area 34,490 km^2
City Radius in CIV 5 GEM should be interpreted as Megalopolis Area (see Wikipedia).
If You want to have a world map where City Radius Area (36 Hex) can be interpreted as Metropolitan Area, You need a map which is around 36 times bigger than Giant Earth Map.
=> 1.080 x 540 = 583.200 Hex
=> 510.000.000 km^2 / 583.200 Hex = ca. 875 km^2 / Hex
=> 875 x 37 = 32.375 km^2
875 km^2 is about the area of a circle with radius 16,7 km (=100 km / 6).
Typical Strategy Games have Map Sizes of 10-20.000 Tiles for a WorldMap.
Please note :
If you increase map scale (length, width) by 6, you must also either increase number of turns by 6 (time scale) or you must increase movement points per turn of units by 6 to keep the original balance of the game.
Some more numbers :
Circumference (equator) : 40.000 km
Modern Destroyer : ca 60 km / h = 1.440 km / d -> 40.000 km / 4 weeks
A destroyer with enough supply on route can travel around the world in around 1 month.
=> 180 movement points per month in GEM, 1.080 movement points per month in higher scale GEM when timescale is realistic.
