Large or Small?
When you have a smaller face, then you need to pick a beard style that matches this. A really large long beard with a small face can look wrong, so you are more inclined to predict proportion, or like a garden gnome. For small faces, consider small detailed beards otherwise, see your face will likely be swamped.
If you are one in the larger headed in our midst, and that I imply that as an alternative to metaphorically. On the other hand, you have to consider proportion. A bit beard can make your head look larger than, rather like an overblown balloon, as someone looking at you will see this large head, with small fiddly detail in the bottom in the face, that is to be lost towards the casual observer. Remember with both large and small heads, you take into account the typical shape also, and apply the style accordingly, with the issue of proportions in mind.
Round or Square?
For your rounder face, you can use the beard to elongate out your shape with the beard longer at the chin. Keep your sides short with length with the chin, and you will probably give the look of an extended face. This is beard shaping process which you new beardos need to pass.
Having a square face you wish to trim with the edges a bit, so applying the converse principle from the round head style of beard, you need it thicker at the sides and shorter with the chin. Sometimes long mutton chops can help lengthen the facial skin. You desire the transition from your side to the chin to be as subtle as possible, so grade it gently. A goatee will work, but don’t make it narrow, and remember to retain length in the chin.