To work out which clothes size is most suited to your figure it is best to take the following measurements.
Make sure you take actual body measurements, not over your clothes.

Chest: measure around the fullest part of your bust and accross your shoulder blades at the back.
Waist: measure the natural waistline, just above the hip bone.
Inside leg: measure from the crotch to where the trouser is normally worn on the shoe.