In Edgar Allan Poe's short story "The Pit and the Pendulum," one of the greatest obstacles the narrator faces in determining the dimensions of his dungeon is the fact that he is in complete darkness. There isn't one speck of light. He is in such complete darkness that he states, "The intensity of the darkness seemed to oppress and stifle me."
A second obstacle to determining the dimensions of his cell is that the room is in reality much larger than he had imagined it to be. Groping through the darkness with arms extended, he had made it all the way to one of the dungeon's walls. He then felt upward along the wall to see how high the wall was, but the wall proved to be too tall for him to feel a ceiling to his dungeon. He then tore off a piece of his garment to attach to the wall so that, as he continued to grope, he could find a beginning point that would help him calculate the dimensions of the dungeon, but the room proved to be even too large to take on that task. He describes the difficulty he has in undertaking that task in the following passage:
I staggered onward for some time, when I stumbled and fell. My excessive fatigue induced me to remain prostrate; and sleep soon overtook me as I lay.
Hence, reasons why he is unable to calculate the dimensions of his dungeon is that it is too dark; it is too large; and he is too weak from having been tormented.