In your save folder there is a folder called "data." Inside that is a file called "Fortress.dat." This is where minecraft checks for the "Structure Bounding Box" that SevenSidedDie referenced in his answer, which I think is correct. Therefore it's possible to see exactly where Minecraft considers a valid Nether Fortress spawning space, and even change it!
If you open Fortress.dat with an NBT editor (I used NBTExplorer, now often considered the best) you are given a list of all the Nether Fortresses in your world thus far, organized by the chunk they began generating in. As you explore more of your world, this list gets bigger as more are generated. Generation starts with the "seed" of the fortress, which is a single room, and then it expands from there, adding conjoining rooms by following some algorithm.
Inside of each fortress are different tags. There is a "chunkX" and "chunkY" tag, which gives the chunk that the beginning of each fortress is in. the "id" tag identifies this structure as a nether fortress. The "BB" tag gives the coordinates of the entire range of the fortress, in the format "X1 Y1 Z1 X2 Y2 Z2." The entirety of the fortress fits inside this bounding box. However, special mobs don't spawn anywhere inside this box, for that you have to go to a higher resolution.
Each fortress is made up of many different segments, as others have talked about, such as a "stairs" or a "corridor crossing." Also in the fortresses folder, along with the other tags I described above, is a List called "Children." Inside of this are 50-200 entries, each representing one of these segments, each with 4 or 5 entries each. They are 'GD,' 'O,' 'id,' 'BB,' and optionally 'seed.'
Now, I don't know what 3 of these entries mean, but I do know what I think are the most important. 'id' describes what type of segment it is, such as "NeSC" for maybe "Nether Stair Case" or "NeBCr" for "Nether Bridge Crossing" (This type of segments is the one you want for farms, as they are the largest, being 19X19) There is always one "NeStart," this is the seed from which the rest of the fortress grows on generation.
BB also describes the bounding box in the same nomenclature as above. Worth noting is that if the BB is "0 0 0 4 4 4" the spawnable area is actually a 5X5X5 box, as the range is inclusive, as far as I can tell. Also, I've heard, but can't confirm, that these are coordinates relative to the region(a 32X32 block of chunks). I was near the origin, so the coordinates corresponded exactly to the generated fortress in my world and where wither skeles would spawn, but if you go farther away, these BB coordinates would remain in a 256 integer range when the real world coords could be much higher.
Warning: If you attempt to add another bridge segment to this list, your game will crash, as far as I can tell. What I needed to do was fix part of my fortress that stopped spawning wither skeles for some reason after I upgraded versions, so what I did instead was simply edit the coordinates of another segment to where I needed. After I reloaded the world (necesarry!) wither skeles spawned again whereever I made one of these segment BB tags encompass.
Might not have been the clearest, but hope that helped!