Im not so sure or maybe i dont understand what you're saying. Shadows arent textures. They're the product of shadowing pixels. At most, you'd stretching a texture, but shadowing its made after skinning and its dependant on the geometry of the object that is casting a shadow. So the stretching of the texture doesnt affects shadowing at all. Specifying the resolution of the shadowmap its basically saying to the engine how often samples geometry that casts a shadow, the more resolution, the more closely follows the geometry of the object. Then you apply a bluring filter or something like that so they look more natural.