I think that the basic definition of "very large" is a starting point. From here, many other ways of describing "colossal" can be developed. One would be "mammoth," a way to bring forth this idea of being "very large." Another synonym would be "massive," helping to convey the idea of large, of great size. In this realm, I like the word, "gargantuan." It is so rare that one gets to use the term in a sentence, but as a synonym for "colossal," it works well. I think that another synonym that might not get the exact word, but captures its intent could be "awe- inspiring." The idea that something is colossal helps to bring to light that it is awe- inspiring. The other synonyms could be "giant" or "huge." These help to bring to light the idea that "colossal" is something intensely larger than other elements, helping to bring to light the idea that all else is dwarfed in its presence. When seeking out synonyms for such a term, it is essential that this idea is sought or embraced in constructing similar patterns of recognition to the original word.
Colossal may imply more than "huge". Let's try these synonymns which have a similar connotation: monumental, gigantic, titanic, expansive.