I'm working on a human model, and so far I'hv been using a single color for the skin texture, which obviously doesn't look realistic at all... Anyone having some tips or tricks to make a realistic - or just semi-realistic (animation-style-realistic - skin texture? Or maybe you know a place, where I can download a texture-picture big enough (Very high definition) to cover a whole body.
Welcome to the Blender community. If we want to make a texture that is high enough resolution to work on a body, we would usually use either Substance Painter or Substance Designer, or if you're really in a rush Bitmap2Material. Algorithmic software is, far and away, the best stuff on the market, and you can even get it free if you are a student.
For your purpose, there are two strategies I would consider.
One, you could import your completed model into substance painter and build/use the default smart material to create a convincing skin shader. You will be able to change things like the color, subsurface color, normal intensity, etc, without having to make your own smart material. Be sure to bake out a world position normal and set the projection mode to tri-planar mapping to avoid seams.
The advantages to this approach is that there will be no seams on your mesh, but you will have to be certain the geometry is final, and this step will have to be repeated if you change your model or your unwrap.
Or, you could use noise maps and seamless mathematical functions with Substance Designer to create a seamless texture with whatever resolution you want and map it onto your model with seams.
Every time, professionals will choose the first strategy.
As of now, there is no way to do what you described in Blender.