Quagmire

Quagmire, the realm of Nightmares, is the plane of Oblivion that is associated with the Daedric Prince, Vaermina. Everytime lightening strikes reality shifts to another horrifying vision, as desrcibed by Morian Zenas. It could be anything, from a dark castle, to a den of ravening beasts, a moolight swamp or the coffin where you see yourself buried.

Knowing this we can then say that the realm is limitless, when the lightening strikes anything from your own fears could be conjured up, and therefore each vision could possibly be experienced by that one person. But we do know that the visions get more and more terrifying, and therefore mortals aren't likely to last here for very long.

Not much more is known about this realm, seeing as it chaotic and ever changing it is unlikely that daedra actually reside here.