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Okay, I did some searching and found this: Actually Poser content seems to prefer 4000x4000 maps. This is because Poser version 6 for Mac can only go up to 4000x4000. Other versions can do 4096, so to stay compatible with more versions, content makers use 4000.

Bagginsbill would probably know if this isn't the case though.

Ahh well thanks for the explanation. Sounds plausible.

@Michael314:
There is no "implied distortion" if the UV  map doesn't have distortion to begin with. If your UV  map's vertex points are as close to the relative proportions of the mesh's vertex points, there will be no distortion in the texture when applied.
However, not all texture makers are created equal and can cause texture distortion all on their own in Photoshop by stretching or sizing parts of a photo too much when trying to make it fit over the UV map. Some people are better at it than others, and the UV  map can only use what it's been given, for better or for worse.
Another thing that can cause distortion is a texture maker failing to consider the natural stretching that will occur on the mesh due to mesh topology. They have to realize that they're looking at a 2D representation of a 3D model and adjust their texturing methods accordingly. You also have to take into account body parts that are going to be morphed or moved that will cause the texture to move along with them, and plan ahead for that as much as possible.

This is why you see so many Poser textures that are really good and so many that are just plain bad.

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