which details does Kafka use to create a stifling, claustrophobic feeling in the opening pages?

Gregor is in a small room which is where most of the action in the story is confined to. This room becomes a stifling metaphor of the Hell Gregor is in. Gregor can only lie there as the walls seem to close in on him. The room becomes both his prison and refuge. Here he suffocates in his terror, torment and fear. Notice the "unsettling dreams" that Kafka begins with. Even his bed becomes a stifling metaphor of a space he can't move his body in.