There are many examples of how power is portrayed in John Steinbeck's novel Of Mice and Men.
First, Curley is a character who believes that he is more powerful than he really is. One could almost picture Curley as a man who has Napoleon Complex (which refers to men who have an inferiority complex based upon the fact that they try to act "larger" than they are). When faced with conflict, Curley tries to play off that he is actually more powerful than he is (unless he is squared off against Slim). Curley's power is not given to him by the other men; instead, he simply asserts it over them.
Slim, on the other hand, is regarded as powerful by the others ranchers. (This, by far, is a much more realistic power.) Slim does not try to act powerful. does not try to intimidate, and does not throw his weight around (like Curley). His power comes from the fact that he is the best at what he does and the other men respect him for that.
Curley's wife is the most powerless character in the entire novel. First and foremost, Steinbeck fails to even give her a name. Through this, she is completely stripped of her own identity and given one connected to a man, Curley. Outside of that, she does have a small amount of power over the men on the ranch. Given she is the only woman with an active role in the novel, she female prowess has some power of its own. She is feared by the ranchers simply for what Curley would do if he found out that anyone had "messed with her."
Lennie is physically powerful. As a bigger man, with diminished mental capabilities, Lennie does not know his own strength. At one point in the novel, he crushes the hand of another man. Outside of hos physical power, Lennie maintains some power over George (based upon the fact that George promised to watch out for Lennie).
George has a very specific power of his own. He is required to be tough (powerful) based upon the fact that he must look out for Lennie. (Without George, Lennie would not be able to survive.) It takes a very strong person to care for someone like Lennie.