Shamneko

Goddess Pandora was sent to Earth to collect "gifts" that were spread among humans. She has a nice body that can temp anyone who looks at her, but when she uses her powers, she shrinks to a young girl's body. The only way to gain her power back is to kiss her reluctant peon, Aoi.

From Terrad: Sae and her friends from school make the "Magic-Users Club", where they try to learn magic (usually ending in some horrible/funny mess-up). Note: Image at MangaUpdates is wrongly attributed to this series and belongs to the alternate story. Please do not replace it.