There are no universal rules for clothing size.
Sizes vary from brand to brand, country to country, even decade to decade.
Brands use "vanity sizing" to keep the numbers on tags low. That is why Marilyn Monroe was a size 14 in the fifties, even though she would be a size 8 in stores today.
Measurement Guide
SHOULDERS
Measured straight across the highest point of the shoulders (from seam to seam if applicable).
BUST
Measured straight across, from armpit to armpit.
LENGTH
Measured from base of the collar or neck, straight down the center of the back to the bottom hem.
WAIST
Measured from top of the waistband, straight across from edge to edge.
RISE
Measured from crotch seam to top of waistband
INSEAM
Measured from inside of pant leg, from crotch to bottom hem.