For shirts measured in width/length, you can find your right fit by doing the following: Grab a t-shirt you already own that fits how you like and lay it out flat on a smooth surface Measure the width of the shirt from armpit to armpit Measure the length of the shirt from the highest part of the shoulder to the bottom hem Use these width and length measurements to select the size you want using the size chart For infant bodysuits, measure the length from the shoulder to the bottom of the onesie when it's snapped.