As a beginner, the general rule is to measure the distance from the floor (in front of your toes) up to your belly button and that will give you an approximate diameter size. It is usually around 36" - 40", so if you are particularly short or tall it may not work exactly.
Another thing to keep in mind is the space you are using inside the hoop. The more space you take up inside the hoop the bigger hoop you'll need. A bigger hoop makes it easier to spin on body - so if you are very petite you will need a smaller hoop, if you are larger then add a couple more inches to your new hoop size as it'll give you more time for each hoop rotation.
As you progress you will probably want to start doing some off body moves, a big and heavy hoop is great to learn on body moves (like on your waist) but can become tiring very quickly when doing off body moves (like on your arms). If you are thinking of learning some new off body tricks then go for something a little smaller and lighter, definitely not anything heavier than a 3/4" HDPE. A good size for a beginner would be anywhere between a 30" - 34" (depending on your height).
The size here refers to the width of the tubing. The most popular sizes are 3/4" and 5/8" hoops, 3/4" being slightly bigger than 5/8". If you are getting a bigger hoop (33" and above) it would be best to pick 3/4" tubing as the smaller tubing may be quite flimsy when doing certain moves. 5/8" tubing is super lightweight which makes it perfect for more advanced hoopers.
7/8" HDPE hoops are quite big and best suited for absolute beginners with very large hoops or people wanting to use a weighted hoop for exercise.
11/16" polypro hoops are in between the 3/4" and 5/8" sizes. It is also denser than both these sizes making it slightly heavier but also more responsive to certain tricks.
All hoops are measured from the outside of the hoop.
Polypro vs. HDPE
Most beginners will start with an HDPE hoop due to it being slightly heavier than polypro. Polypro hoops can be very lightweight (especially the 5/8" tubing) and are more suited to intermediate and advanced hoopers that like to go at lightning speed!
Other than the weight difference, the HDPE tubing is more softer than the polypro. This makes the HDPE more "bouncey" and also more durable, polypro is more likely to crack or bend in extreme temperatures. The choice of polypro or HDPE tubing usually comes to personal preference and depends on what you are planning to do with the hoop.