You needn't worry about it. Wearing sunglasses won't dent your head. You can just buy sunglasses according to your own face shape and frame size. Just buy sunglasses that can flatter you. And as long as you won't buy sunglasses that is too tight, it won't dent your head and face. Just try some light weight sunglasses.
Even though I am not quite understand about your question that sunglasses may dent your head, I am quite sure that the answer is no. Wearing sunglasses will does no harm to you head. According to my understanding, there may be two possibilities sunglasses may "dent" your head. First, the sunglasses are too heavy that it may press your eyes into skin? That is good imagination but I do not see it happens. It is known to the whole world that gravity is downside. It may add pressure on you nose instead of on your skin. Second, you may worry that the sunglasses legs may leave dent on your head? Oh, that is possible. But the only reason cause the problem is because the sunglasses are too small for the wearer. It is so tight that there will be dents on head and wearer will feel uncomfortable. Get a pair of sunglasses of appropriate size will prevent the problems.
If you choose the right size of the sunglasses, they will not dent your head. The sunglasses should be with the suitable nose pad which will make you wear comfortable. I suggest you to go to the regular eyeglasses store to buy the pair of sunglasses where the client will know which type is suitable for you.