The chanel sunglasses must cost you a lot of money. If it is not suitable for you to wear, you could adjust the nose pad to make it tight in the structure by using the screwdriver. You could also go to the real stores to let the client help you do this because you may screw the parts of frames and lenses.
Well, as far as I know, there are certain glasses that are able to be adjusted to various head sizes. However, it seems that your Chanel sunglasses, despite its great quality and perfect design, is not suitable for you. Anyway, you can roll up some napkin around the frame. I recommend you ask for replacement from the seller, typical, they would agree to replace another pair.
Well, generally speaking, you will feel uncomfortable when your channel sunglasses are too loose, and they often slid off from your eyes. At this moment, you should try some ways to tighten it. First, you should Put on your eyeglasses and, and look in the mirror. And then you just need identify precisely where the frame problems occur. Then the important step is coming. You should tighten the screws on your eyeglasses. Besides, for the sake of safety, just adjust the arms of the eyeglasses in a careful, slow way. Also, the tool such as a pair of needle-nose pliers is necessary. Finally, you can make a small, corrective bend in one arm at a time. Anyway, you can just have a try. But on the other hand, I will suggest you console it with a professional person.