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The Cave Troll Action Figure from the the Lord of the Rings movies is actually nearly the correct proportions for a Terrasque.
About he Cthlhu model..is it the Vinyl one? I purchsed it 3 years ago but i have never put a model together so it currently remains in its Box ( but I have put together the old Pewter Ral Partha Mini of Cthulhu).
WotC has not announced any plans to make figures larger than huge at this point. I do not think unlikely, however, that they will release some specific gargantuan or colossal figures at some point. For figures that large, I suggest looking for alternative sources. More than likely, a plastic dragon from some other source may end up being cheaper and more readily available ...
Not gargatuan or colossal, but I've recently picked up DeathSleet* from Reaper to use as a mature black dragon. The horns go the wrong way (back instead of forwad) but otherwise it's pretty servicable as a huge dragon, and a heck of a lot more intimidating than the "large" black dragon that WotC put out on the Dragoneye.