When it comes to describing something that is big, there are many different synonyms that can be used. The phrase "a big one" can be substituted with terms such as "a large one," "a giant one," "a colossal one," or "a massive one." For a more colloquial approach, you could also use phrases like "a whopper," "a mammoth," or "a behemoth." Additionally, adjectives like "huge," "enormous," "tremendous," or "monstrous" can be used to describe something that is of considerable size. Overall, there are many different ways to describe something that is big, and the choice of language will depend on the specific context and intended meaning.