Besides the discussion of "base size", I would rule that raging is a class ability that is lost when transformed into a dragon.
Also I'd like to add:
Demonic Bulk is a supernatural ability gained from the Abyss bloodline whilst raging, and Transformation magic(in regards to the Form of the Dragon) specifically states:
While under the effects of a polymorph spell, you lose all extraordinary and supernatural abilities that depend on your original form (such as keen senses, scent, and darkvision), as well as any natural attacks and movement types possessed by your original form. You also lose any class features that depend upon form, but those that allow you to add features (such as sorcerers that can grow claws) still function. While most of these should be obvious, the GM is the final arbiter of what abilities depend on form and are lost when a new form is assumed. Your new form might restore a number of these abilities if they are possessed by the new form.