How does the narrator of "The Pit and the Pendulum" decide to figure out the size of his cell?
Let me give you the quote from the story first, then explain it:
I tore a part of the hem from the robe and placed the fragment at full length, and at right angles to the wall. In groping my way around the prison, I could not fail to encounter this rag upon completing the circuit. So, at least I thought: but I had not counted upon the extent of the dungeon, or upon my own weakness. The ground was moist and slippery. I staggered onward for some time, when I stumbled and fell. .... Shortly afterward, I resumed my tour around the prison, and with much toil came at last upon the fragment of the serge. Up to the period when I fell I had counted fifty-two paces, and upon resuming my walk, I had counted forty-eight more;—when I arrived at the rag. There were in all, then, a hundred paces; and, admitting two paces to the yard, I presumed the dungeon to be fifty yards in circuit.
What he decides to do is walk around the wall, counting the paces and calculating that every two paces is a yard. He uses a piece of cloth from his robe to mark the place where he started, so that he knows when to stop. In total, he walks 100 paces, which tranlates to 50 yards.
This incident lets the audience know that the narrator is intelligent and calculating, and foreshadows that he may be able to "figure" a way out of his current situation.