Jesus asked this to show that worry accomplishes nothing. "Taking thought" here means anxious care, the same worry He has told them to give up (Matthew 6:25). No amount of it can change what God has set. Barnes says it simply: "With all your care you cannot increase your stature a single cubit. God has ordered your height." Gill makes the same point, that it is impossible for a man, with all his care and thought, to add a cubit to his stature, or, as he prefers to read it, "to his age." The question takes away worry's false promise of control.

No one can, and Jesus does not wait for an answer. He moves straight to the next worry: "And why take ye thought for raiment?" (Matthew 6:28). Luke's account spells out the conclusion in another question: "If ye then be not able to do that thing which is least, why take ye thought for the rest?" (Luke 12:26). Jesus ends this teaching by telling His hearers to leave tomorrow to tomorrow: "Take therefore no thought for the morrow" (Matthew 6:34).

A cubit was a measure of length. Barnes explains that it "was originally the length from the elbow to the end of the middle finger," not far from 22 inches. The word translated "stature" is helikia, which can mean either age (the length of one's life) or bodily height. Thayer places this verse under the first meaning, "term or length of life," while noting that others take it as height. Barnes reads "stature" mainly as height ("This word means height"), though he adds that "The original word, however, means oftener age," and if so, "a man cannot increase the length of his life at all." Gill argues for the sense of age, since Jesus is speaking of something very small (Luke 12:26); Matthew Henry thinks the measure of a cubit points to bodily stature. That is why the ESV reads, "add a single hour to his span of life."

The verb behind "taking thought" is merimnao, to be anxious, the same word Jesus uses in Matthew 6:25, 28, and 31. Both Barnes and Gill point to Psalm 39:5, where David says, "thou hast made my days as an handbreadth."

This question puts worry in its place and God in His. Our height and the number of our days are in God's hands, not ours. David prayed, "LORD, make me to know mine end, and the measure of my days" (Psalm 39:4), and confessed that God had made his days "as an handbreadth" (Psalm 39:5).

Matthew Henry points out that every grown person has already received what worry could never produce: "An infant of a span long has grown up to be a man of six feet." God did that without our anxious help, and He can be trusted with the rest. Henry's counsel is that "we must reconcile ourselves to our state, as we do to our stature."