From my understanding there will 2 main servers at the start, those being NA and EU, with possible other servers after that depending on demand. Now I don't really know how the server itself will limit sizes since the game will be on cloud servers which can expand and shink on the fly.

With that said, lets talk about the size of the first continent map, it's going to be Huge. Morbis MIA describes it as 'stupidly big.'

Map Size Link: https://www.youtube.com/watch?v=g6Aa6gVOvwg

From my understanding they are hoping to keep the population around 200k (100k players and 100k NPCs) though as most things are subject to change.

Now, since we are on the subject, you might want to take a look at a video covering the SpatialOS system. It's very scary what it will be doing and how much this can change the gaming world entirely.

SpatialOS Link: https://www.youtube.com/watch?v=qG6roE_9ERc

I hope this settled any doubts or questions concerning resourses and map size and density.