My take on this is as following:
Wanna wildshape into a Huge-sized Dire Tiger?
Step 1: create a Huge tiger, based on the Dire Tiger in the Beastiary. Up its size, attacks etc.
Step 2: What will its classification be? Since a Tiger of this size hardly would be concidered natural, I'd say it won't be classified simply as a Huge Animal, but rather a Huge Magical Beast or something along those lines.
Step 3: Are you able to wildshape into this new creature? No, because I can't wildshape into Huge Magical Beasts.
This is my train of thought as to why this won't work.
