File:Nether fortress corridor crossing "area bounding blocks".png

Summary
This is the reason of why all of the wither skeleton farms have 17x17 spawning platforms. The base level of the "bounding boxes" is marked by Stone Bricks (only at the crossing). The "area bounding box" of the outermost Stone bricks is marked with the planks (only horizontally [x and z axis] and not vertically). So this adds up to: 6 (the "area BB") + 5 (the "structure BB") + 6 (the "area BB", but on the opposite side) = 17 on the x and z axis.