Will a 5mm down advancement make my face too long?

For a sliding genioplasty, I was told the doctor wants to move my chin forward 5mm and down 5mm. While I don't want my mentolabial fold to be too deep, is 5 mm down too much, or is the tissue itself going to look shorter ? I moved my jaw 5 mm downwards to try to simulate it and didn't like how it looked.