For the transparency, use Format Data Series (right-click) and choose None
as the Area setting.

I don't think you can change the shape, though. Oh, you could use the Fill
Effects in the Area and choose a picture. You'd need to create a picture to
use, though.

Ahhhh. Got it. That actually works. Cool!

On your slide, draw an autoshape -- use a star for this experiment. Give it
no fill and whatever line color you want. Now right-click and Save as
Picture. It should default to EMF; you can leave it at that.

Now open the chart, right-click and select Edit Data Series. Click Fill
Effects in the Area secton. Choose Picture tab. Navigate to your EMF. That
will replace the circles with the stars. You change the size as you always
do in a bubble chart -- by changing the appropriate data.