How does the fact that the snake was extremely large contribute to the story?

The snake's size serves to illustrate Jim's courage and Antonia's pride in him.

‘I never know you was so brave, Jim,’ she went on comfortingly. ‘You is just like big mans; you wait for him lift his head and then you go for him. Ain’t you feel scared a bit? Now we take that snake home and show everybody. Nobody ain’t seen in this kawntree so big snake like you kill.’

She went on in this strain until I began to think that I had longed for this opportunity, and had hailed it with joy.