Killjoy, Lithia doesn't mean a literal sphere. The idea is that if you go far enough in one direction, say, for instance, east, it'll eventually just place you at the far west of the map, and have you go east from there. It's not literally a spherical world, it just has the same effect in the sense that you can go one direction forever and keep passing over the same spot.
Like Jetra says, this is called wraparound.
EDIT: Also, it looks like WorldBorder has a wraparound setting, but it doesn't sync up the chunks, it just works by straight-up teleporting people.
Though, it occurs to me someone might be able to make this work pretty well by... painstakingly copying the chunks near the edge. Almost definitely not worth doing that, though.