The analyses of a text #2

"You're quite a sailor, I suppose?" I said to Em'ly. I don't know that I supposed anything of the kind, but I felt it an act of gallantry to say something, and a shining sail close to us made such a pretty little image of itself, at the moment, in her bright eye, that it came into my head to say this.
"No," replied Em'ly, shaking her head. "I'm afraid of the sea."
"Afraid!" I said, with a becoming air of boldness, and looking very big at the mighty ocean. "I an't!"
[David Copperfield by Charles Dickens]
I'd like to know if "it" refers to "to say something."
And I'd like to know "very big" means "with open eyes."
Thank you in advance for your help.