I've got these myself and know from experience on how to check there sized but here's a site that shows and explain's it better here, click this link> serebii.net
Encase in the future you wonder, does it's size pass down whilst egg breeding... YES it does Check this this; click this link> breeding gourgeist.
My experience with Gourgeist, you can find them on Pumpkaboo on Route 16 but this Pokemon requires trade evolution to evolve into Gourgeist. You'll need to catch a box full of these Pokemon and then check there cries. Trade Evolve them and check there sizes. with the links above.
Small Size: 2'4" (0.7)
Average Size: 2'11" (0.9m)
Large Size: 3'7" (1.1m)
Super Size: 5'7" (1.7m)
Small Size: 20.9 lbs (9.5kg)
Average Size: 27.6 lbs. (12.5kg)
Large Size: 30.9 lbs (14kg)
Super Size: 86 lbs (39kg)