I always pictured thought speak as basically the same thing as e-mail, you could direct it to one person or 3 or 1000 depending on how you felt.
As for the distance, it seems to vary, like someone already said, in book 2 for instance rachel could barely hear tobias, in a tree outside chapmans house, when she was in the basement. But like TMP already said, on an aircraft carrier a diffrent floor is mostly likely a huge distance away, but Ax could have also been right above him. It would be nice to come up with a yard range for about how far it could be "heard" my vote would be about 50 yards max.
I also pictured a thoughtspeak voice as the same kind of voicover thing that we have in movies.