尺子 (chǐ zi) is the common everyday noun for a physical ruler (measuring instrument), used with the measure word 把. 尺 (chǐ) can also mean 'ruler' but is more often used as a classifier for length (a Chinese foot) or in formal/technical contexts; for a physical ruler, 尺子 is the usual choice. 尺度 (chǐ dù) is an abstract noun meaning 'standard' or 'criterion', not a physical tool.