You've got a judgment call on the part of the story guide to add restrictions that aren't in the printed rules. because there's nothing in there (at least that I recall) saying that you have to see where all of the target is.
That's what I thought - and I agree completely, part of any SG's job (and the job of the Troupe) is to make sure things don't get "out of control" just because the Rules don't mention something.
thnx.
Go for broke and add 2 magnitudes more, 60000 mile wall. OR, keep it 6 mile and make it 10 times thicker and 100 times higher. :mrgreen:
... and a moment, later, the mage was crushed as the wall hit him square in the back... 
:mrgreen: