They are very small, with an outside diameter of approximately 12mm, Inside 11mm.
18 gauge wire a little thicker than standard ear wires which are normally 20 gauge - so if you have sensitive or tiny piercings these may not be for you!
They DO require twisting to wear - You will receive them twisted in the OPEN position. Simply slide into piercing, rotate into position, and then gently twist closed until both ends of the hoop meet/line up, as seen in photo.
Photo with US quarter shows these hoops in the 'twisted open' position.
These are intended for semi-long term wear - frequent twisting for taking on/off will weaken the wire over time and they will break.