You shouldn't apply a tiled skin texture to a 3D model of a human at any scale.
First off, skin is not uniform. A great many things about a person's skin vary over their body:
Shade: Whether stark as in a bikini tan, blended as in a farmer's tan, or subtle as in the variation between face and body on most people, no one has uniform skin shade, except perhaps full-time nudists.
Oiliness: The forehead and nose will be shinier than most other parts of the body.
Pore size: They aren't the same size over the body's surface.
Wrinkliness: Not just face wrinkles, but also knuckles and knees and sag lines and...
Interfaces: The skin color changes at all the body's orifices and ducts.
Skin thickness: Your pinna is a bit darker than the cheek right next to it due to the thinness of the skin, allowing more of the blood and underlying tissue to show through.
Secondly, UV maps generally aren't perfectly uniform in their mapping of 2D polygon size to 3D polygon area. If you apply a 2D texture to the UV map uniformly, it will stretch out where the UV map uses a proportionately smaller polygon as compared to the area of the corresponding 3D polygon and pinch in the inverse case.
While it is possible to define a UV map in such a way that the polygon areas do map uniformly, you typically only want to do that for a geometric sort of model, and often not even then.
If you're texturing a model of an ottoman, a uniform UV map may make sense. You can stuff the UV map fairly tightly while still keeping all the pieces rectangular, and without distorting anything.
For models as complex as a human body, you typically don't want uniformly detailed textures to begin with. People do not pay equal attention to all areas of another person's body, so we should not give all areas of the texture an equal amount of space in the UV map. Even if you're working on an artistic nude, you simply don't need as much texture detail between the buttocks as around the eyes.
As a rule, you want the most detail in the face area, followed by the hands and other key parts of the body. For most works of art, the UV map polygons for the bottom of the feet can be tiny: they'll either be unseen, or seen so briefly that we're not going to need a lot of detail.
When texturing a human model, it is common to break it up into a few major areas, each with its own UV map. (e.g. Head, Torso, Arms, Hips, and Legs.) These 2D UV map files are typically all the same size — e.g. 2048×2048 px — even though the pieces of the model they belong to have different amounts of surface area. This is how we want it, because different areas of the model need different amounts of detail. Applying the same texture to all of these UV maps would be wrong, even if you are using UV maps with an equal 2D-to-3D polygon area ratio. The source texture scale must change to match the UV map scale, else the parts will not blend together properly.
Bottom line, you should hand-paint such a texture.
One technique for applying a flat skin texture file to a model at varying scales is to use the clone stamp tool along with the Clone Source panel. The tool lets you copy textures from one file to another, even when the destination is a texture in a 3D layer. The panel lets you change the scale of the clone source without changing the source file.