Spandex, Lycra, and elastane all refer to the same fibre — Lycra is a brand, spandex the American term, elastane the European one. A little goes a long way: 2% adds comfort stretch to jeans, while 10%+ creates the cling of activewear and bodycon.
For sizing, more elastane means more forgiveness — a stretchy garment fits a wider range of bodies, but it can also bag out over the day as the fibre tires.