If the width specified is 2.0, the point will be a square of 2 by 2 pixels. In OpenGL, the term line refers to a line segment, not the mathematician's version that extends to infinity in both directions. The easiest way to specify a line is in terms of the vertices at the endpoints. An extraneous vertex is ignored. Draws a line segment from the first vertex to the last.
Lines can intersect arbitrarily.
With OpenGL, the description of the shape of an object being drawn is independent of the description of its color. When a paraticular geometric object is drawn, it's drawn using the currently specified coloring scheme. Until the color is changed, all objects are drawn in that color or using that color scheme. Polygons are typically drawn with the pixels in the interior filled in, but you can also draw them as outlines or a set of points. In OpenGL, there are a few restrictions on what constitutes a primitive polygon.