(Slave Leia is problematic when taken out of it’s story context as a position that Leia is forced unwillingly into by Jabba. The difference between Padmé and Leia wearing sexulized outfits, is consent. Padmé elects to wear her revealing clothing Leia does not. See article for further analysis: The Washington Post: The Fraught History Of Princess Leia’s Infamous Bikini.)
It is rare to have actual measurements on costumes from movies, however, in the book, [Star Wars Costumes: The Original Trilogy]. Leia’s slave bikini that she wears in Return of the Jedi at Jabba’s palace, appears in the book next to a ruler. The costume items seen in the pictures are not aligned perfectly on the ruler so a little figuring is required to get actual measurement. These pieces of course are measured to fit Carrie Fisher, and would have to be adjusted for every individual Leia. If you feeling like doing the math, these measurements plus knowing how tall Carrie Fisher was, can give you pretty exact proportions on the original costume. and from there adjust the costume to each individual Leia’s needs.
My measurement figuring comes out to roughly 8″ 1/2 inches for the bikini bottom at the top, (which in the photo is at the bottom of the picture, against the ruler). Leia’s armband is roughly 5″1/2 inches in length, measuring from one the curling end to the other.