The best way to measure a crewneck, hoodie, t-shirt, or tank top is to take one of your existing shirts, lay it flat, and then measure the width and length.
To measure the width of tee shirts, crew necks, and hoodies, measure side seam to side seam at 1" below the sleeves.
To measure the length of tee shirts, crew necks, and hoodies, measure the shoulder seam at the connecting point of the collar/hood, ending measurement at the bottom hem of the shirt.
To measure the chest of outerwear, measure armhole seam to armhole seam at 1" above the arm hole ending. Double measurement to solve chest measurement .
To measure pant inseam, determine the length starting at crotch point ending at ankle.