Ok so this is a bit harder to explain but i'll try:
The "map size" is space. Like,all of the space they have unlocked. You can sit in a part of the unlocked space with absolutely nothing around you for hundreds of thousands of kilometers. The entire galaxy is the "map" at any time.
Now basically what the game does is create an "instance bubble" around your ship. Everything that enters this bubble is part of your instance. Your instance can hold 32 players (and no idea how many NPCs, tons of them) at one time. Say I just sit in space somewhere and by sheer coincidence you drop out out super cruise (getting to that in a bit) close to me. We are now in the same instance bubble and can interact. We might be 100km apart but we still exist in the same space.
Now of course the game needs to do some stuff to make it actually possible for you to get to places within your lifetime. Seeing how the universe is 1:1 that might otherwise take a while. XD
So what the game does is that there are star systems. And you can select them and use your warpdrive to warp to the star of the system. This is pretty much a few seconds of being in witchspace and then out you drop. When you drop out of warp,you are in what is called "super cruise". In this mode, your starship flies slower, at the or even faster than the speed of light and all planets, stations and points of interest in the system are marked for you. You can then fly to wherever you want in the system, trying not to overshoot (this can happen XD ). Once you are at a specific speed and range to a marked point you can drop out of super cruise and the game will kinda give you a hand and put you close to where you wanted to go. So say you dropped out at 800km from a space station you had marked, the game will drop you 20km from it cause it's impossible to do it by hand otherwise. You CAN however drop out at those 800km from the station, this is your decision. You can also drop out of supercruise at ANY TIME wherever you are and you'll sit there, in normal cruise in that exact spot in the system where you dropped out. Once implemented, you'll be able to use this exact system to "interdict" people in super cruise and pull them out of it to ...well do whatever you want with them. You can also meet up with other people wherever you want, doesn't have to be by a station or POI.
I hope this kinda explains it, if not, ask away and I will explain.