Don't underestimate how important it is to make sure that you choose body jewellery which has a suitable wearable area.
Barbells are commonly used in eyebrow, tongue, cartilage and nipple piercings. The wearable area of a barbell is the length of the bar between the balls – not the length of the whole piece of jewellery. If your barbell is curved, for example for your eyebrow or PA piercing, you need to measure the space between the balls at its shortest point (the inside of the curve). A barbell shouldn't fit so tightly that the balls press into the skin – there needs to be a bit of room for movement.
Labret studs (and their pal the monroe stud) are used in various locations of lip piercings and are often also worn in tragus piercings. The wearable area of a labret/monroe stud this is the length of the stem between the ball and the disc.
Ball Closure Rings (BCR's) are a different kettle of fish (fish – piranha – fish! Get it!). The wearable area of a BCR is the internal diameter of the ring. Same goes for circular barbells and segment rings. The larger the gauge of the ring, the more important it is for you to get this right – otherwise you will end up with a piece of jewellery that could be a canny big bigger than you were expecting! Owch!
Some types of jewellery don't really have an option to choose from, such as nose studs and body spirals, which are a law unto themselves and are both pretty flexible and universal in terms of their fitting.
Ear stretching jewellery divvied up into plugs/tunnels and expanders/tapers. Lets deal with plugs/tunnels first. They are pretty much the same externally, but the important bit is whether you are looking at a saddle or a flare. A saddle has a bit more flexibility in its wearable area as it doesn't come to an abrupt halt like a flare. However, you dont want your ear to be hanging over the edge of your saddle to be too prominent! Some flares can be pretty squared off, particularly acrylic/surgical steel and screw fit styles. Either way, the true wearable area for plugs and tunnels is the measurement from the front to the back (not the gauge as many people assume). Many styles wont offer you an option, whereas some brands, such as Kaos Softwear, offer a number of wearable areas to choose from. Generally the wearable area will get smaller as the gauge increases.
Expanders/tapers offer the best flexibility of wearable area an expander goes on and on and on, so as long as your ears are ridiculously massive, they should fit pretty much any expander of the correct gauge without it flapping around too much. You can always use a couple of (or just one) o-ring so keep it in place.
I've probably missed some, but this covers all of the basic styles of body jewellery out there. Choose carefully!