小 (xiǎo) is the basic word for 'small' in size, and it also means 'young' when talking about age (e.g., 小孩 child, 弟弟小 younger brother). Use 少 (shǎo) for 'few' or 'a small number' – it's about quantity, not size. For 'young', 小 can be a prefix before a name (like 小李), while 年轻 (niánqīng) is a separate adjective for young people.