A single squared area in StarCraft is 32 pixel wide and 32 pixel high. All buildings in StarCraft have a span of at least 64x64 pixels. Even though the buildings appear to fully occupy the area as you are choosing where to build them, the vast majority do not. They have gaps. These gaps are extremely annoying to deal with for new players trying to wall-in; experienced players learned to deal with it through continual trial and error. Once these gaps are accurately measured however, it's possible to predict whether a certain wall-off combination will block a specific unit or not before even trying them.