I'm a 20/22 US W with a G cup, and this "one size" is loose with room to spare. Laid flat: the arm hole is 10in/25.5cm long, the side seam beginning under the arm hole is 15in/38cm long, the body is about 28in/71.2cm wide between side seams, with an overall length of 48in/122cm from shoulder to center hem. I'm sure it varies between garments, but the three I have are all close to this.
The side slits come up a little high if this isn't worn as a beach cover-up, but not so much to flash off your bits. If you're worried about it, wear boxer briefs, chub rub shorts, or even tights. I wear these kaftans everywhere - you can't stop me. The cotton is light weigh and breathable but woven well. There was minor color bleeding but only on the first wash with the darker colors and not dramatically. I wash them in a cold water load and hang dry. This is a great find!! Pockets would be my only improvement.