A "cubit" was a little less than half a meter. In this instance, it is used as a metaphor for adding time to a lifespan. (See: [[rc://en/tw/dict/bible/other/biblicaldistance]] and [[rc://en/ta/man/translate/figs-metaphor]])
## And why are you anxious about clothing? ##
This rhetorical question means "you should not be worried about what you will wear."