Yes, do be careful on the size issue. I've been in several weddings and the stores have always told me "Oh order the next size up just in case..." In case of what? I'm a size 4. One place told me I needed a 10. I laughed and ordered the 4. I didn't need any alterations. That's how they screw you anyhow.
I was MOH in my cousins wedding...all of the party was different sizes, and the dress was a two piece with an elastic waistband. I got mine sent in from out of state, took it to the local tailor and they took out the elastic, and threw on a zipper. The rest of the party was pissed...they wished that they had thought of that.

Elastic waistbands are not a good look.
Anyhow, my MOH was my cousin and she was great, but then I was pretty easy going and she lives out of state so she didn't really have to do much. On the day of the wedding and at rehearsal she ran interferance as we were out of the country and people were starting to drive me nuts asking me 8000 questions and wondering why we weren't getting married where they wanted us to be. She basically kept the irritating people away on that day so that my husband and I could just enjoy the day.