How big the size of Dyntopo Detail size is?

Lately I've been sticking to 7px for relative detail size. Remember it's determined by how close you are to the object you're sculpting. If you're zoomed out then the polygons will be bigger. If you're zoomed in the polygons will be smaller.

I don't recommend trying for perfect smoothness with dynamic topology. Your sculpt will become too heavy and cause Blender to lag. It's more about achieving the shapes and silhouette well more than it is about never seeing polygonal evidence.