The size chart shows the approximate size of the customers who usually buy that size, based off my own experiences fitting customers. However, there is no such thing as a standard clothing size and they do vary from brand to brand. As such, I highly recommend checking your measurements.
If you're new to waist training: go one size down from the top size you normally wear. If there is more than one size in your range, pick one near the middle. e.g. if you are a size 12 and new to waist training, the Size L will probably be best for you.
If you are experienced at waist training: You probably already know your tolerance, and may want to pick the trainer at the tighter end of your size range.
If you've just had a baby: Pick the trainer at the looser end of your size range - and check with your doctor, too!
If you have a narrow waist and wide hips, or find your size naturally fluctuates throughout the month: I recommend getting two trainers in different sizes so that you can alternate them depending on your needs.