Apple Motion 2 Tutorials - Manual (Page 199)

Tutorial 4 Working With Particles 199
To change the pattern of the emitter:
m Choose Outline from the Arrangement pop-up menu.
Next, make the circle smaller and center it in the Canvas.
To change the size of the circle emitter shape:
1 Move the Adjust Item tool until it is hovering over the circle shape.
The circle shape becomes highlighted.
2 Drag the pointer inward to decrease the size of the circle.