There is no one lengthy description of Curley in the book. So, you need to piece together what Curley is like by various descriptions. For one thing, Curley is known to be a good fighter. He did some boxing in the past.
The old man looked cautiously at the door to make sure no one was listening. “That’s the boss’s son,” he said quietly. “Curley’s pretty handy. He done quite a bit in the ring. He’s a lightweight, and he’s handy.”
Curley is also short. In fact, he has a complex because of this, and he feels the need to prove himself. So, he is easily angered and fights others.
Well . . . tell you what. Curley's like a lot of little guys. He hates big guys. He's alla time picking scraps with big guys. Kind of like he's mad at 'em because he ain't a big guy. You seen little guys like that, ain't you? Always scrappy?
One of the climaxes of the book will be when Curley messes with the wrong guy. He fights Lennie and pays dearly.
“Sure,” said George. “I seen plenty tough little guys. But this Curley better not make no mistakes about Lennie. Lennie ain’t handy, but this Curley punk is gonna get hurt if he messes around with Lennie.”