Hoop size is a personal preference and really depends on what you're used to practicing with, and the type of tricks you enjoy. The most popular sizes used in studios are 35 and 37 inches for adults. A smaller hoop can be easier for beginners to maneuver, and a larger hoop is great for tricks that require more headroom. You may ask your instructor what hoop size you're used to using in the studio for a reference.
We really recommend measuring to make sure you get the right size hoop, but the sizing on the hoops is a little flexible and up to personal preference. Most aerialists like the hoops to be big enough to sit in without hitting their head (though, there is a tiny hoop trend on social media right now). To measure for your standard size, you can sit in a chair and measure from your bottom to the top of your head and add two inches. Measuring is important as people carry their height differently (some have longer legs, while others longer torsos which can impact hoop size). But, as a general guideline, people under 5'0 will choose a 31-inch hoop, 5'1-5'3 may choose a 33 inch hoop, 5'3-5'5 may choose a 35 inch hoop, 5'5-5'8 may choose a 37 inch hoop, and 5'8-5'11 may choose a 39-inch hoop. We have temporarily discontinued our 41-inch hoops but most people under 6 feet can use a 39-inch hoop.