Whether it's a monumental effort requiring vast amounts of strength or a monumental cruise ship with eleven floors and five sets of elevators, the word monumental describes something imposing or massive in size.
Monumental can also refer to something that's of outstanding importance or significance. A monumental decision for you would be one that changes your life — like the day you got engaged or when you decided to move to New York to be an actor. The word monumental can also be used if you're talking about a monument, a statue or structure commemorating a person or event. You might admire the monumental stonework on the statue in the town square.