What point is Douglass making in paragraph 2?

He seems self-conscious about speaking in such a great hall.

The papers and placards say, that I am to deliver a 4th [of] July oration. This certainly sounds large, and out of the common way, for it is true that I have often had the privilege to speak in this beautiful Hall, and to address many who now honor me with their presence. But neither their familiar faces, nor the perfect gage I think I have of Corinthian Hall, seems to free me from embarrassment.