Using D&D's "1 inch = 5 feet" scale, and taking into account that a bipedal (eg. "Tall") monster is Gargantuan at 32-64 ft high, and Colossal at 64+ ft high, you should just be able to measure your Cthulhu and know.
If he's 12.8 inches or taller, he's Colossal. If he's between 6.4 and 12.8 inches, he's Gargantuan. I'm thinking Colossal, personally.