"Amplitudinous" is not a common word, but it means great or extensive in size. There are several synonyms for this word that can be used interchangeably depending on the context. Some of the synonyms for "amplitudinous" are extensive, spacious, broad, vast, wide, roomy, capacious, and large. These words describe things that are big or have a lot of space. They can be used to describe physical objects like a house or a field, or more abstract concepts like a person's knowledge or emotions. Using these synonyms can add variety and depth to your writing, and help you to be more precise with your descriptions.