On what page in the novel The Known World is Skiffington's map discussed?

You can find this discussion on pages 174 and 175.

We can tell that Skiffington's map is important, because the title of the book first pops up as a phrase in the text--"the known world"--while the characters are discussing his map:

"A Russian who claimed to be a descendant of Waldseemuller had passed through the town and Skiffington had bought the map from him. He wanted it as a present for Winifred but she thought it too hideous to be in her house. Heading the legend were the words 'The Known World.' Skiffington suspected the Russian, a man with a white beard down to his stomach, was a Jew but he could not tell a Jew from any other white man."

So it's definitely a good idea to take a look at this conversation, and to consider Skiffington's map and what it might reveal about him as a character as well as what it might reveal about the book's theme.

Specifically, Skiffington's map is hanging on the wall of the prison. The characters talk about how the map has some serious problems! For one thing, Florida isn't on there. And for another, the entire continent of North America is the wrong shape. So as a faithful representation of the world and its layout, it's really not that useful. One of the prisoners, Broussard, offers to get Skiffington a new map. "That one will do fine" is Skiffington's reply. 

What we learn here is that, even though Skiffington could easily get a more accurate map, he doesn't care to. He likes his map the way it is. The fact that he clings to the incorrect map helps illustrate how Skiffington clings to other incorrect ideas and misconceptions about the world.

Skiffington's understanding of "the known world," therefore, is just as skewed and flawed as the map that he adamantly holds onto. He's afraid to change, or to admit that the world is different than he sees it. His understanding of the world is hundreds of years outdated and just plain wrong.