I would assume you meet the inside of the apartment and outside, on the window ledge. The setting inside the apartment is regimented, normal, and safe: even the window to the outside is difficult to push open, to have the slightest hint of freedom. It doesn't "budge," so he has to "jolt" it open. Tom is so familiar with the place that he can tell where his wife is standing based on the sound of her voice: it is "muffled" at one point, so he immediately knows that she is in the bedroom closet. Nmbers safely equate to numbers, even in the construction: The Beneckes pay "seven and a half dollars less" than their neighbors, due to having a slightly smaller living room.
Outside, on the ledge, is called "the darkness." The putty on the outside window frame is "dried out and brittle," suggesting the dangerous condition of the unknown; it could collapse into chaos at any moment. The ledge is "not quite as wide" as Tom expected. When he almost falls, it is so terrifying that he must concentrate on "holding on to consciousness" to survive. The wind "snatch[es] up his cries" when he screams for help, in contrast to how easily he heard his wife's muffled voice earlier. He visualizes his apartment: "warm, cheerful, incredibly spacious."
Reread the story, using the above for guidance, and look for more contrast between the two settings and what they represent.