I actually do a combination of both. I get the idea of what I want for a larger, world map first, then I pick a random location to put a continent, and I build from there. For me, by doing this I get a bigger idea of what goes on in my world and so I can have potential material to write in later. As of now, each of my continents serves a purpose for some form of story telling, and each one has a story to tell that can go into great lengths. I guess though, the big question I ask myself is, "How far do you want to take this particular piece of work?" By that I mean, is this a story that I want to use to build more stories off of, and in turn, world build around? Or will it be a one shot deal that I can set aside once it's finished?

In my case, all of my writings so far will span my world, and from that I do a combination of making a world map, then making the continents to see how everything can fit together. As I go through the process and learn more about cartography, I make changes based off of what it is the world should be like. For example, the biggest question someone asked me, and in turn I asked myself, is do I want a realistic fantasy, or a straight made up world? From how my friend described it, by doing a realistic fantasy, even though it is fictional, it should have plausable elements that can be explained in some shape of form other than, "magic did it." With straight fantasy, it doesn't matter, you can do as you want.

Once I figured out that I wanted realistic fantasy, I started to build my world, constantly figuring out the proper placement of rivers, mountains, tundras, landmasses, etc. I know that turned out a bit long winded, but I hope that helped?