My 10- and 12-year-old nieces are using 37" hoops. The 12-year old is an inch or two taller than me (I'm 5'2") and the 10-year-old is a couple inches shorter than me. I feel like 37" is a good size that fits them at the age they are now but as they grow they likely wont need a larger hoop.
Great thanks! This will be a good beginner hoop for her :)