Beginner Hoops: are made from polyethylene (or poly) which is irrigation tubing that can normally be found at your local hardware store. This tubing is thicker and heavier which makes it easier to learn how to hula hoop around your waist, along with several other tricks that can be done on the body. Regarding size, the heavier and larger the hoop, the slower the spin - the easier it is to keep rotating.
The general rule of thumb is to measure from the floor to your belly button. That measurement is the height in which your first hoop should be. This is only a general way to measure. We know that all body types are different and that one general rule is not going to work for everyone. Keeping in mind that the larger the hoop, the easier it is to keep on your body, but also remember that a hoop that is too big will be harder to learn off-body tricks with.
Another great article regarding hoop size can be found here.
Polypro: are made from Polypropylene tubing which is a light and bouncy tubing. Polypro hoops are great for all skill levels as hoops can be cut in to larger diameters for the early beginner and cut down for well-seasoned hoopers.
3/4" polypro is about the size of your thumb, whereas 5/8" polypro is about the size of an average pointer finger. The smaller the size, the faster and bouncier the hoop.