All soul shards behave like vanilla spawners in terms of where mobs can spawn. Technically, the size is 8x8x3. Make a 7x7 box around it, and then determine corner in which both x and z decrease when you walk toward it. Extend the box to 8x8 in that direction. The feet of the mobs can spawn at the level of the spawner, as well as one below and one on top of it, so make your ceiling two higher than the spawner (assuming you are farming a two-high mob). I recommend, however, blocking off the two blocks directly above the spawner so that mobs don't get stuck there. Of course, you can also do 9x9 if that's simpler to build.
Hope this helps!