The size guide ia here to guide you, and if needed how to measure yourself to avoid disappointment with online shopping.
Take measurements over underwear while standing up straight. The tape measure should fit snug around your body without being too tight or too loose. Hold it just comfortably and horizontally.
Bust: measure around the fullest part of your breast and across shoulder blades. Let your arms hang down.
Waist: measure the thinnest part of your waist and don't hold your breath.
Hips: measure around the fullest part of your hips.