Well, below assumption is based on these two screenshots of a french site that had sg. like 26 new pics:
First is a setup of a game with:
a, the russians as played civ
b, with a huge map
Second pic is a strategic view of a gmae that has the Russians as played civ
My assumption is that it is the game that was set up in the first picture.
So we can see how big is a huge map, which is the biggest we can play.
Let's count:
On the second picture we see 32 colums of tiles, each is about 11-12 long vertically.
That makes the number of tiles on the picture ca. 350-380.
Now we have to estimate the multiplier for this to get the whole map.
If we check the minimap on the second picture, we have to decide whether
the whole map is with the white lined edges (ie. bigger version) or without it (smaller one).
I guess it will be the smaller version, as I believe we can see ice tiles below the lined edges. That can be the end of the playable map there...
If so, then we can estimate the multiplier:
Something like 3 both vertically and horizontally, which makes in turn:
9 * 380 = 3420
ca. 3500 tiles as a huge map...
(with the lined edges version min map: ca. 4500)