We made a shipping spotlight some time back with some great feedback from our members. I went with the uline.com 2mil poly bags and just picked a size thought would be appropriate.
I would HIGHLY recommend the Kordon Breathable Bags. I have shipped hundreds of coral with complete success. They are a little more expensive than a typical plastic bag ($0.15 per bag) but since this bag requires very little water and very little space, you save money or boxes and shipping fees!
Gary http://www.novalek.com/kordon/articles/breathing_bags.html
You will be double bagging.. this is a must and even some corals you will want to triple bag... it is not about a single bags thickness but about redundant fluid containment should the first bag lose integrety... 3mil and 4 mil bags are great for this... Corals come in many sizes so it is important to have at least two options a 6"X12" works well for single corals and a 12" X 18" is good for larger colonies... the depth is important to make it easy to tie the tops LOL I purchase my bags from Uline www.uline.com they are fast, cheap and high quality
Heck If you need a few LMK on a micro order and I'll just add some in the package for you 50? 100? save you the costs of the bags and get micromussa instead