If you order a full length dress, we usually add in the heel height of the shoe to the dress length, so that it looks 'full length' when worn with heels.
Example:
If your height is 168cm, a full length dress may be about 20cm shorter, making the dress about 148cm. If you wear 8cm heeled shoes with the dress, the you will still want the dress to touch the floor, the length should then be 148cm+8cm = 156cm.