Cells come in a variety of sizes. Bacteria are among the smallest (0.0079–0.012 inches [0.2–0.3 millimeters] in diameter), while cells of plants and animals are generally larger (0.39–1.97 inches [10–50 millimeters] in diameter). Cellular size is determined by the surface-to-volume ratio needed for substances and enzymes to complete their functions within a cell.