Mike, I have been using the Sig 4" bell cranks in everything that is larger than a Ringmaster.
  When I use the Sig b/crank I set up my controls where the front leadout is the up line.
   These cranks are supposed to be self centering this way also.  I really like the way that they work.
    I have read that in using a 3" crank you can alow it down by drilling a new pushrod hole 7/16" from the pivot post.  Never have tried it so I can't speak from 'speriance..
      Hope that this helps...

I have a friend (who shall remain nameless to protect the guilty) that uses 3" bellcranks no matter what.  On being cajoled into flying a beast with a Saito 72 in it, I found that the Netzband wall* is alive and well.   On any hard maneuver, the line that was not pulling belled out behind the one that was, and control was definitely limited.   Clearly, the control surfaces were overloading the amount of force the narrower bellcrank could provide.

* Netzband wall - the condition where the control system does not have enough power to drive the surfaces due to the aerodynamic resistance generated.  This totally limits the control input possible, no matter the surface travel available.  Bill first published this concept in one of his articles.

I use at least 2.5" bellcranks in 1/2A models, and prefer 3".  On a .15 or larger model I would put in a 4" if at all possible.  Slop in the system is less critical, and control forces are more easily dealt with, not to mention any friction or hinge stiffness.  If you can fit the bigger bellcrank, I know of no reason not to use it.