Its original size was "L (12' tall or wide)". Looking at the most similar creatures:
A mimic (Large) can have almost any dimensions, but usually is not more than 10 feet long. A typical mimic has a volume of 150 cubic feet (5 feet by 5 feet by 6 feet) and weighs about 4,500 pounds.
A phasm (Medium) in its natural form is about 5 feet in diameter and 2 feet high at the center. Swirls of color indicate sensory organs. In this form, a phasm slithers about like an ooze and can attack with a pseudopod. It weighs about 400 pounds.
So maybe....
A protein polymorph can have almost any dimensions, but usually is not more than 12 feet long. A typical protein polymorph has weighs about 6,000 pounds.
The remaining 3 skill ranks could go into Search or Survival. Maybe Track would make a better 3rd feat choice, since "protein polymorphs are voracious hunters and must devour food regularly to provide the necessary nutrients", and it is as intelligent as an average human?