Choosing a dress is stressful in many different ways. This is one of the best reasons to have a Maid of Honor, or, in my case, a sister who has recently tied the knot as well.
The first reason why you will collectively lose your mind is that, no matter what size or shape you are, you probably won't feel very beautiful while looking for your dress.
You know people will be looking at you, lots of people that you love, including the love of your life, and that there will be photographic and possibly video featuring you as the main attraction. It's like being a real princess for one day with none of the experience real princesses have developed since age four in dealing with this kind of exposure.
Due to my above reasons I will be, and I suggest you do as well, choosing a very comfortable and flattering dress that will not require a girdle. I will have enough to worry about with out being able to breathe well or sit down comfortably.
FlyAwayWeddings.com offers a very simple guide to choosing the right dress to fit your shape.
HenryRoth.com offers a more advance guide including information on accessories and bouquets.