We rented from cruiselineformal and had a local place take DH's and DS's measurements. They (the local place messed up really bad--the girl at the rental place caught it when I called to reserve.) I ended up just looking at DH's suit and shirt sizes in the closet, and gave her DS's boys clothing size. Their boys sizes fit very true to size BTW.
I would look at EBay for formal wear for the girls, or depending on when you are sailing, after CHristmas and after Easter there are usually GREAT sales on party/formal type dresses in the department stores.
I wore two different outfits. A long gown for formal night, and a tea length skirt and off the shoulder lace top for semi-formal Same shoes and bag for both.
DH wore his suit on semi-formal, as did DS. I would have loved tos eem them int the "monkey suits" again, but didn't want to push my luck LOL!
Anne