Thanks for your answers. However, they are not responding to my question exactly.
To be more precise, I would like to have, for example, one background layer in 100% zoom and one frontground layer in, lets say, 800% zoom. In that way, I could trace 8 times smaller 1px wide contours of an object and its details, still keeping all the sharpness and exactness of the initial sketch as a guideline.
@Pix3M: The sketch does not have to detailed at all. It is rather matter of shapes, which I am good drawing at, than the excessive details. Resizing down the sketch, however abstract (that is, without details) it might be, destroys its lines. Anyway, thanks on your advice.