The writer speaks of his lifespan is if it could be measured by the width of his hand. AT: "only a very short time" (See: [[rc://en/ta/man/translate/figs-metaphor]])
This simile states that the length of the writer's life is so short that it does not exist. This is an exaggeration to stress how short it is. AT: "the length of my life is barely any time at all" (See: [[rc://en/ta/man/translate/figs-simile]] and [[rc://en/ta/man/translate/figs-hyperbole]])
Life is short, and the psalmist speaks of it here as if it were only as long as the time it takes for a person to breathe one breath. AT: "The time that humans live is as short as a single breath of a person" (See: [[rc://en/ta/man/translate/figs-metaphor]])