As the cell size increases the surface area per unit volume decreases. Therefore, the surface area would not allow materials to enter or leave the cell quickly enough to meet the cell's needs. As a result, most cells are microscopic in size. Cell size is limited due to the inability of very large cells to provide nutrients and water and remove wastes in an efficient manner. The size of a cell is limited by the relationship of the cell's outer surface area to its volume or its surface area-to-volume ratio. Thus, the correct answer is option A.