How much can the scalp be stretched to allow for a skull implant? (photo)

I have a very small, flat head as well as a slanted forehead likely due to some form of plagiocephaly or lack of natural plate expansion.Together they make it look like my head consists of only my jaw. Because of that I am looking to undergo skull augmentation surgery to the entire top of my skull. I've included an image to show the area where augmentation is needed. My question is how many millimeters can the scalp be expanded to allow for a silicone implant without a tissue expander? Thank you