I wouldn't feel bad asking her what size she is; she had to expect the question at some point during this whole adventure. I picked out the dresses I wanted my bridesmaids to wear and have let them go and try on their own sizes; one being a plus size and one being a smaller size. If for any reason they can not arrange to wear what I initially chose then I will just look for another option. When it all comes down to it, my dress is the main focus....
Good luck with everything.