you need to assign a size to the Actor as well, not just the Canvas.
the size of the Canvas is independent of the size of the Actor to which the Canvas is assigned to, as you can assign the same Canvas instance to multiple actors.
if you call:
a.set_size(300, 300)
you will see the actor and the results of the drawing.
Clutter also ships with various examples, for instance how to make a rectangle with rounded corners using Cairo: https://git.gnome.org/browse/clutter/tree/examples/rounded-rectangle.c - or how to make a simple clock: https://git.gnome.org/browse/clutter/tree/examples/canvas.c