For a question such as this one, you'll really need to read closely into the play's stage directions. Of particular importance is the scene description that precedes scene 1, where Tennessee Williams describes in depth the layout of the porch. Note that all the play's action is concentrated in this one location (a fact that is specified by Williams himself), so these physical details are all extremely important in establishing the look and setting of the play. Williams explicitly makes note of its "Gothic" qualities, stating that its features create an impression "not unlike a doll's house."
However, even as the play's action is confined to this one porch, you can still see traces of the larger rustic setting that surrounds it. In scene 1, for example, we observe Jake racing out of the house to set the fire at the Syndicate Plantation, and as he does so, Williams's stage directions make note of such details as a barking dog, the roar of the car, and (later) "a muffled explosion" in the distance. His stage directions include additional details such as the sound of locusts and the mooing of a cow. This culminates in the sound of numerous voices, involving people never seen within the play. Even when reading the play as text, this creates a very stark sense of isolation, as Williams takes great pains to give hints of this larger world that extends outside the porch, even as the setting remains entirely limited to its confines. This has the result of making the play's atmosphere all the more constrictive in effect.
Meanwhile, time is also a major component to the play's setting and visual design. Indeed, each of the play's three scenes takes place at different parts of the day. Williams's stage directions are specific on this account and should be read closely. For example, scene 1 opens with the following:
It is early evening and there is a faint rosy dusk in the sky.
Compare this to the note that opens scene 2:
It is just after noon. The sky is the color of the satin bows on the window curtains—a translucent, innocent blue.
Finally, you should look at the stage directions that open act 3, taking place at night. In this respect, the time of day (or night) itself becomes an important component to the play's underlying atmosphere and look, critical in the creation of these effects.