The first code is actually drawing part of a spiral. Change the size value to 0.99995 to see this. What is happening is each recursion of foo is drawn a little smaller, and so the distance ("y 1") on each recursion is a little less each time. As it curves, as it is going less each time, the curve makes an inward spiral.
The second example makes a circle as the size stays constant each recursion. So, the example draws a many many sided regular polygon. The example, as you no doubt know, recurses forever.
The last example draws an infinte line. Each recursion draws a square just south of the last. This keeps going forever. As the drawing progresses, you see the line get thinner and thinner as the camera "pulls back" to show you the ever expanding line. Eventually, the line is so thin there is nothing to draw. It is all less than 1/3 of a pixel. Hence, you don't see anything.
You should register on the cfdg gallery site and post your avatar design there.
2.0beta v9 is old. The latest release version is 2.0 v12. The latest beta version is 2.1beta. 2.1beta adds a tiling capability that allows you to create repeating cfdg images for desktop or web backgrounds of 3D object textures.