It's important to try to do as much as you can with large strokes. This practice keeps our eye on the big picture. Save details for later if they are needed at all, and keep them minimal. Too much detail in the wrong place will pull the eye away. Know where it is you want the eye to linger. On portraits it's usually the eyes, for instance.
Use small brush strokes for details in the highlights.
We don't need to show much detail in the shadows. The shadows are there to support the form and if placed properly can help achieve strong composition and graphic design. Details will show most at the edges of highlights where the planes are angled just slightly away from the light.