Hoop Sizes
My hoops and bigger and heavier than the children's hoops for sale in some shops. This makes them easier to hoop with. My standard hoop is made from tubing with a diameter of 25 mm. I also make lighter hoops from tubing with a diameter of 20 mm which may be more suitable for smaller hoopers, or hoopers who bruise easily, and also for hand hooping.
Hoop Diameter
The common recommendation seems to be that when a hoop is held vertically, with the bottom edge touching the ground, the top should come somewhere between your navel and your chest.
Most people find a hoop with a larger diameter easier to keep up, because you don't have to rotate it so quickly to keep it from falling down.