Here's my explanation.
You can think of it as a face-centered cubic array of chloride ions with an interpenetrating sodium ion lattice.
(Adapted from wps.prenhall.com)
In the above diagram, there is a chloride atom at each corner and a sodium on in the centre of each edge.
The distance from the centre of a
Thus, if the distance between the ions is X pm, the edge length is 2X pm.