Is there any evidence that people can see any of the Daedric Plane(t)s from Nirn? Or, is there any mention of Daedric Planets on the whole? On another note, would the size of the Planet as comprehended by a mortal mind be larger depending on the size of the Prince? Example: Clavicus Vile's Planet would be larger than, say, Peryite's.
The Daedric Princes do indeed have Planes, but they are not "plane(t)s". The difference, as far as I can tell, is that their homes/entities are composed of Oblivion, where as the God-Planets are inside the void but not of it. Since there is no circular planet-like visualisation for the Daedra, it seems reasonable to assume that they are infinite and cannot be thought of as bigger or smaller than each other.
Planets
are infinite. You have to remember that the perception of a planet is an illusion caused by viewing one infinity encompassing another. Since infinite oblivion is encompassed in infinite aetherius, I don't think it's unreasonable to assume that they would appear spherical to the mortal eye (if they could be perceived from Nirn). Oblivion just looks like space, since it was created as a reflection of the void outside the Aurbis. The Daedra themselves can't be made of nothing, so their realms are their own selves, as they are the only sources of matter/creatia in Oblivion.
For ages the etada grew and shaped and destroyed each other and destroyed each other’s creations. Some were like Lorkhan and discovered the void outside of the Aurbis, though if some saw the Tower I do not know, but I know that, if they did, none held it in such high esteem. In any case, some of those that did see the void created its like inside the Aurbis, but each of these smaller voids sought each other out. Void shall follow void; the etada called it Oblivion. What was left of the Aurbis was solid change, otherwise known as magic. The etada called this Aetherius.
They are not really that different from the Aedra. The Aedriatic planes are also infinate. Only Nirn and the moons are finite. Hey, who knows? Maybe with the right magic/technology you could see right into the realms of the Daedra. Maybe "his realm is obscured to mortals", is meant literally - as in certain planes may been seen through a telescope and others may not. Just speculation.