There is no fixed formula to get the right size. It depends on your skill and style, and if you use it for practice, dance or performance.
For beginners I recommend a heavier hoop, which, if standing up from the floor, would reach your bellybutton. The same hoop can be ideal for meditative sustained spinning, helping to feel the hoop making body contact.
However, a smaller and extra light hoop, such as a polypropylene hoop, can work great if you wish to work on spinning and isolation tricks. If you’re using more than one hoop, lighter hoops won’t be as heavy.