Install my plugin pack viewtopic.php?f=16&t=22819
To make a square around the whole canvas, do this:
Press Ctrl-A to select the canvas, Effects > Selection > Outline Selection
(Adjust size and color)
For an Elipse, use the circle selection tool to draw your selection, refine with the Move Selection tool, then Outline Selection.
Now, if you just want to draw a circle, make a selection, then use Effects > Render > Polygon / Stars. Just set "Verticies" to maximum. Adjust brush width, select antialiased, and you should be all set.