Pumpkaboo gets the same size as it's mother. If you breed male with ditto the Pumpkaboo will get the same size as it's father.
///Jofly
If you're breeding a Female Gourgeist, then the offspring will inherit the mother's size. If it's a male Gourgeist with a Ditto, the offspring will inherit the male's size.
Source: experience, The Pokemon Academy's video
