Hii Jonatan,
Have you not done the Fundamentals of Sculpting? https://cgcookie.com/courses/core-fundamentals-of-digital-sculpting-blender
Relative Detail is in (screen) pixels, so if you zoom in on your model, the details you sculpt get smaller; if you’re zoomed out, your (sculpted) eye might cover 12 pixels, so a detail size of 12 px will result in Edges that are as large as that eye.
