A lot of people, certainly a lot of GMs I know, start simple or small and add details as they go.
Maybe it's a single town with a dungeon near by or the starport and some alien ruins but as time goes on it will become a town near a city that's in a kingdom on a continent, etc.
I don't always (usually) work that way. I try to. I wish I did. No, I start huge and epic and have developed the rather odd skill of learning to edit and hold off and pull the camera in tighter.
While thinking about my upcoming Traveller game for example, I came up with some such idea that reminded me of Red Dwarf (the British Sci-Fi Sitcom that I can't believe I haven't mentioned more often on this blog as I love it so much). That somehow got me looking up pictures of ships from the show on Google, which lead me to thinking how much the Omega Planet Skipper from EXONAUTS' site would fit a slightly more serious version of that universe, not to mention Space:1999, which got me thinking of the InSpace/Space:1999 idea I had and BOOOM!
Picture a Traveller game in which the players start out as the typical Space Truckdrivers/Serenity(bleh) group of ner do well troubleshooters when a misjump or some other accident sends them millions of years in the future.
The last Humans alive in the universe (perhaps) with maybe an android or uplifted animal companion for extra fun, they have to maintain their ship and stay alive just to find out if there is any life in the universe not entirely hostile to them. Maybe they want to find out what happened to Humanity. Maybe they want to find Earth. Maybe they want to find some tropical planet somewhere and just lie on the beach forever drinking cool drinks with umbrella's in them.
Refining it, the idea works best as Red Dwarf/InSpace, with big, high concept science fiction like Solaris, Forbidden Planet and 2001: A Space Odyssey crossing paths with the occasional remnant civilization of genetically engineered creature or renegade robot left behind by a dead-beat Humanity.
Personally I think the concept is awesome but I also thinking it would be hard for my current group to 'get' the idea. I can't be too tongue-in-cheek with these guys even though we joke around a lot. They like their gaming serious and aren't adept at understanding my dramatic comedy or comedic but serious game ideas.
I now have to start widdling this down to see if I can use any of it with them or just scrap it and start from scratch.
Hmmm...or I could just save it for the New Jersey Group. Ah, Jersey.
Who said that?! >_<
OK, OK...something less amazing...