Tiamat

Tiamat represents the evil side of chaos and is the quintessence of water and war.

Although she has a physical body, her size is not measurable due to her location in the Abyss. She sleeps between the bottom of the portion of the Abyss that lies in the physical plane, and the mouth of the Abyss that lies in another plane. As she takes up much of the space in this region, it would be impossible to view her in a manner as to make any form of size comparison. Still, there is a reasonable assumption of her appearance based on lore.