The size of the wire used for hoop earrings really depends on the style of hoop you desire to create. If you want to make plain hoops that go through the ear with nothing on them, take the size of a piercing into consideration and use from an 18-gauge for larger holes, down to a 21-gauge for smaller.
If you want to make a more substantial hoop, then my advice would be to use any gauge you want, from an 18- through a 14-gauge, and attach the hoop to an ear wire or post for the customer's comfort. When using these larger gauges, it is fun to forge the hoops with a chasing hammer and a bench block, to add some cool texture to the metal.
For a start, check out these Free Patterns for Hoop Earrings.
Answer contributed by Dale "Cougar" Armstrong