void glLineWidth(GLfloat width);Sets the width in pixels for rendered lines; width must be greater than 0.0 and by default is 1.0. The actual rendering of lines is affected by the antialiasing mode.
You can obtain this floating-point value by using GL_LINE_WIDTH_RANGE with glGetFloatv():
float val[] = new float[2];
gl.glGetFloatv( GL.GL_LINE_WIDTH_RANGE, val );
System.out.println("min="+val[0]+" max="+val[1]);
Keep in mind that by default lines are one pixel wide, so they appear wider on lower-resolution screens. For computer displays, this isn't typically an issue, but if you're using OpenGL to render to a high-resolution plotter, one-pixel lines might be nearly invisible. To obtain resolution-independent line widths, you need to take into account the physical dimensions of pixels.