Hoops usually come in 2 thicknesses ( 20mm which we call the lightweight OR 25mm which we call the heavy).
The more experience you have the smaller and lighter your hoop gets. 

The hoops that I use are 20mm thick with a diameter of 30-32". BUT when I started hooping I used a 25mm thick hoop with a diameter of 42"... It took months to move down sizes...

If you are considering ordering a hoop please do not hesitate to ask for advice or just to have a go with the hoops I have even if you are not ordering from me. I would not get too stressed out about the exact size but it is good to make sure that the hoop you are getting is suitable for your fitness level, size & purpose.

I do not deal with glittery, sparkly, metallic looking hoops, only grippy & practical hoops that make the beginnings of the hoopdance experience a bit smoother & more confident :)