The size of each realm is assumed to be considerably large. It is revealed in the story/Konquest mode in Mortal Kombat: Deception. In this game you get the chance to explore all of the different worlds and you can just free-roam and see what's going on in them. Obviously some realms are a lot smaller than others, like Outworld is considerably larger than other realms like the Orderrealm and others. But I assume that the realms are actually a lot larger than the sections we were able to explore, since they obviously can't program a whole big world into one game in the matter of time they had.

Earthrealm map:



Netherealm map:



Orderrealm map:



Outworld map:



Chaosrealm map:



Edenia map:

There's also the Earthrealm Village as the first place you're in during MKD's Konquest. So in the game, the Earthrealm is the widest. But it's rather obvious that they're only segments of the whole realms.



As for the "real" size of these realms, I think it's safe to assume, that the foreign ones are about the size of the "Realm of Earth". But just how big is our realm?

well i cant see all the realms being limited to just those maps, but i do know however that the nether realm is the largest since its infinite. its contatntly expanding because more an more ppl are wntering it