To know your true shirt size you will need to take measurements of your chest, neck and waist. For more fitted shirts, you should also take measurements of your arms. Once you have your measurements, use the size guide provided by your retailer to see how your measurements determine your size. Remember, it is always better to size up than to size down if you find your measurements are a little over. For example, if your chest measures 48.5" but your retailer states that their XXL Size has a chest measurement of 46-48" and their 3XL has a chest measurement of 50-52", you will want to opt for the 3XL.