Cassius is making an allusion (literary reference) to the Colossus of Rhodes, a famous statue that was one of the Seven Wonders of the Ancient World. This allusion occurs during Cassius's famous discussion with Brutus during which Cassius states his case against Caesar and argues that Caesar has no more right to become king than any other nobleman. Cassius is speaking sarcastically about Caesar by comparing him to such an enormous and famous icon. What Cassius really means is that Caesar is no more important or capable than any of the Senators in Rome.
Cassius refers to the Colossus of Rhodes, a reportedly enormous statue of the sun god Helios. He compares Caesar to this sculpture at the Lupercal, where he and Brutus hear cheers and worry about “some new honours that are heap'd on Caesar.” In this moment, Mark Antony is offering Caesar the crown, which he turns down three times. It is a marvelous opportunity for Cassius to convince Brutus that Caesar has grown too powerful and popular.
Cassius tells Brutus, “I was born free as Caesar; so were you,” and relates several anecdotes in which Caesar demonstrated his mortality, once by nearly drowning and then by falling ill. When Brutus expresses concern about the shouting crowds, Cassius declares, “Why, man, he doth bestride the narrow world / Like a Colossus.” It is no surprise that the people adore Caesar when they put him up on such a pedestal. In comparison to Caesar’s huge reputation, the world is narrow and the other senators “Walk under his huge legs.” His presence eclipses theirs and places them in the unsavory position of standing at his feet.
In this same speech, Cassius famously says, “The fault, dear Brutus, is not in our stars, / But in ourselves, that we are underlings.” He urges that they take action, suggesting that they have put themselves in this subservient position. Cassius emphasizes that Caesar is simply a man, but he conjures up images of Colossus when he talks of Caesar’s exaltation over all others: “When could they say till now, that talk'd of Rome, / That her wide walls encompass'd but one man?” That man is Caesar, and Cassius points out to Brutus that Caesar is no Colossus.