It may be a good idea to measure the circumference of your dog's muzzle using a sewing flexible tape measure. (cspca.com)
To suss out your ideal size, use a flexible tape measure to measure your body or find a shirt you already have that fits perfectly and measure that. (businessinsider.com)
Use a flexible tape measure and measure around your breasts at the their fullest point (directly over the breasts), and around your back in a complete circle. (nickisdiapers.com)