1 huge, vast, enormous, gigantic, giant, mammoth, massive, gargantuan, Cyclopean, Brobdingnagian, immense, monumental, titanic, Herculean, elephantine, jumbo:
Mystery surrounds the exact methods used in moving the colossal stones used in the pyramids.
2 spectacular, stupendous, wonderful, awe-inspiring, staggering, extraordinary, incredible, overwhelming, unbelievable:
The old Hollywood extravaganzas were described by press agents as 'colossal'. I think you have made a colossal mistake in failing to hire Cynthia.