how can I get the right sized maid dress

okay so I lost a bet now... I have to wear a maid dress to school for Halloween the only problem is I dont really have a small ribcage/waist so I thought id ask reddit. I need help knowing wear to buy it and one thats not to reveling. erm btw im dyslexic and English isn't my first language sorry

ps im about 5'9 170 lbs