Alright, sorry for the delay, I wanted to get home to my computer to type this rather than use my phone because that would have been a huge pain in the ass.
1a. The cause of the disaster that robbed the world of magic for
200 years 300 years (sorry, typo) is currently being discovered by several characters in an on-going quest which you can read about
here and
here. It essentially involves the attempt of a few mortals to stop a power-mad God from taking over the world and the consequences of their actions.
1b. I am concerned that you may have the wrong idea about how post-apocalyptic this world is. Tessanis (the name of the planet) has never reached a level of technology that we would consider modern. This is a medieval setting. The set-back occurred because magic was stripped from most of the world, and magic is what many cultures relied on
in lieu of technology. A scramble for resources caused certain cultures to thrive (Ebonfort) while others perished (Vircastoria) and even more were created in the aftermath (the Kvaren and Pyresia).
2a. I am not sure about the total population in numbers of the entire planet.
@Twhirtley will have to weigh in on this one but you can except populations of cities to be similar to real world medieval numbers.
2b. Right now the only "planet" known per se is Tessanis. There are other "realms" such as the Feywild, Aether, and Abyss that are accessed through magic rather than any sort of space travel. This is more akin to Planar travel in D&D lore.
3. For plot reasons, not all of the cultures or cities of the world have been discovered, but what your character will likely know is that Ebonfort is currently the strongest kingdom around, primarily due to its non-reliance on magic. Frigmount to the north is also strong, but was mysteriously cut off from the rest of the world (the cause being discovered in the quest and RP threads I linked above). There are now rumours of cities in the desert to the south and the west (Pyresia, dragon city, pretty awesome). There is the potential for us to unveil more at any time.
This universe is completely separate from any other so we're not having anyone ported over through space jumps and things like that.
Edit: In addition, since it was touched on earlier, the way we RP here is a bit more sandbox style than others. We don't just have one massive thread that progresses chronologically through time. You are free to start new threads to RP certain events in any place that you like. If it's something that requires GM guidance, we're here to do that. There are also always quests going to explore/discover/fight baddies, etc. These RP threads can be set in any point in time (within reason). To get a better idea of what I'm talking about, check out the character sheets and see how people's RPs are listed at the bottoms.