The Nevernever has been defined with relative vagueness compared to other Dresdenverse elements, in part because the Nevernever generally works at the speed of plot. Getting somewhere using the NN should take only as much effort as is warranted by the plot, or at the very least, NN travel should allow one to travel much more quickly (albeit with exposure to more danger) than traveling in the mundane world.
As far as actual NN geography, what I've absorbed so far is that there are three ranges:
Near Nevernever: maps to real-world spots, but is not in any way a 1:1 landscape duplicate of the real world, and spiritual association is more important than geography; generally considered the Faerie Realm due to the presence of the Fey here; can change over time, but stays relatively constant to mortal lifetimes; time passed differently, but usually in one's favor.
Far Nevernever: more fluid and subject to the whims of the local denizens; time passes differently here, but not usually in a helpful way; this may be where places like Down Below reside. Somewhere here can be found the Outer Gates which bar the Old Ones from this universe.
Beyond the Outer Gates: a plane of madness and unimaginable chaos, locked away from the world.
And then there are Demesnes: usually linked to a single real-world spot, but could be a traveling portal to multiple spots of similar energy/association; most often seen with ghost hauntings, which produce a nightmarish pocket dimension which is adjacant to wherever the ghost happens to be haunting at the time.