1. I do seem to recall that the DMG or the PHB, I forget which, mentions that a DM may disallow the use of your abilities, in his discretion, based on the situation. Does anyone remember where that was?This on the other hand, is opposite of that sound advice. If the designers intended for a rogue to be able to shift any opponent as a power then the player shouldn't have to justify how they can do it any more than a wizard has to justify how he can make someone fall asleep in the middle of a fight. Coming up with a cool explanation for how an ability looks while being used is certainly encouraged but that is quite different from the burden of justification.
2. In any case it is inevitable that an RPG work this way. The rules can't spell out every eventuality, so a DM is appointed to act as referee when dealing with unforeseen circumstances.
3. Finally, I used the example of shifting an enemy because that's what the OP used. I do not think that shifting any enemy of any size is something that needs extensive justification, because I don't find it unbelievable that a small PC could make a very large enemy shift a distance that, to the very large enemy, is actually very small. The general principle, however, applies in other circumstances where a power is being used in a context which is not contemplated by the rules, and doesn't make immediate, intuitive sense.