Unless you have excellent sewing skills, this will be hard to alter. Can you order 2 sizes and return the one you don't actually wear?
Be careful of thinking the whole thing is going to be simple & inexpensive right off the bat - b/c I did alterations for a bridal shop for years and in my own home business (before I went back to school to finish engr degree) and sometimes the altering ends up being a more complex job. Your best bet is to find someone reputable (by word of mouth) in your community who does this type of work out of their home (low overhead) and go from there by having them take a look at it (or a picture of it) and estimate the alterations cost.
I would call around to find someone who can do that for you that does professional seamstress work. They will be able to measure you and then alter the dress in all the appropriate places to fit you exact. It would not cost but about $20 to do so.
I have altered all sorts of silk dresses with boning. It should not be that hard to have it altered. You take in the sides and perhaps the seam at the zipper and bust darts or seams.
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