All clothing is measured laying flat. The width (A) is taken from armpit seam to armpit seam across the front chest. The length (B) is taken from the back collar seam, typically were a tag would stick out, measuring down the front to the bottom hem. The length will only be measured from the top of the back collar, if there is no collar seam.
Sleeves (C) are measured from the shoulder seam to the end of the cuff. If the article does not have a shoulder seam, it will be measured from the armpit seam to the end of the cuff.
Pants and shorts are also measured laying flat. The width (A) is taken from hip to hip across the top. The outseam (B) is taken from the top hip measuring to the bottom leg hem. The inseam (C) is taken from the crotch seam measuring to the bottom leg hem.