One thing I would keep in mind when you order the dresses is to let the BMs pick their own sizes. When I was MOH in my best friend's wedding last year, they told me I needed to order a size 22. I almost always wear a size 14 in dresses, so I was like "WTF?" I thought my measurements matched the 16, but my best friend was flipping out that my dress wouldn't fit (she has some ED issues, whole other story) so I ordered the 18 to appease her. I had to pay an extra $40 for the dress for a "plus size", and then when they came in it was HUGE - I could take it on and off without zippering it. The bridal shop wanted another $150 for alterations. My boyfriend's paralegal is a wonderful seamstress and she altered it for me for a box of chocolates.
We had to take a whole panel out of the dress, it was absolutely ridiculous.