Slimmer with a Beard or Moustache

Man with beard, moustache and turtleneck
Q: Can a beard or moustache make a man look slimmer? What elements do I need to look at for a slimming effect?
A: Facial hair on a man, when it's properly groomed, can dramatically alter the apparent shape of the face. This can make the face look slimmer, broader, longer, etc. This is, of course, due to the creation of lines to draw attention in the direction you desire.
The secret is to remember the rules of volume and line that govern what makes a hairstyle flattering for a face shape. For men, the addition of facial hair can work exceptionally well with a haircut to change to look of the face.
For men with a round face, the facial hair can be trimmed with harder angles to give the impression of a squared jaw. A rounded shaping of chin whiskers can camouflage a pointy chin.
For my own face, which is round, I have my hair buzz-cut to 3/4 of an inch, and wear a goatee/mustache combination called an "anchor", where the goatee extends 1-2 inches to the side and is tapered to a point. The overall effect is the vague shape of a ship's anchor.
The position of the hair on top counter-balanced by the hair on my chin elongates the look of my face, while the shaping of the lower line of the facial hair helps to "square" the look of my face.
Likewise, the type and shape of the facial hair can widen the face. For instance, a man whose face seems overly narrow, could use wide sideburns that flare on the ends to widen the look of the face or use a chin-strap beard to broaden the look of the face.
As with haircuts, the secret is the balance between the cutting lines and the volume of the shape of the hair. A long, puffy beard is NOT going to flatter a round face, just as a close-cut vertical sideburn and soul patch isn't going to make a thin face look fuller. Remember the rules about balance, and you should be fine.
See also:
Men and their beards
Stylish looks for men's faces
Beards and the shape of your face
Basic styles for moustaches and beards