Aero, I look foreword to hearing about your experience.
Reece, the possibilities are endless for sizes and shapes. Smaller beads diffuse more when packed tightly, but create more drag as well. Be creative. Smaller seed beads also move more when they are loose. You can use tiny beads with larger beads or shapes submerged in them, so the vapor blasts through filters of beads to the spaces around the larger ones (minimizing drag, but creating the foam of an exploding 2 liter Cola as it escapes the top layer of 11/0's, OR any other of the infinite combinations you can think of
).
However, I guess that a chart like this one could be helpful:
http://www.landofodds.com/store/seedsize.htm
The links I post are mostly random links I find related to what we are talking about (not necessarily links I support or have used). Your best bet, IMO, is to use them (and the ideas I share) as jumping off points and search a bit (please share what you find too).
I have shared some ideas for different ways that beads can be arranged to create different degrees of diffusion, action, drag,...in this thread and others (before there was enough interest for this thread to be created).
Toke it easy (and full of seed beads).