For your reference, BY/DJO's earrings sizings are divided into 4 categories:
Mini : less than 1cm
Small : 1cm to 1.2cm
Medium : 1.3 - 1.5cm
Large : 1.5cm and up
Sizings for the other accessories can be found on their each individual pages.
BY/DJO's earrings are made of various materials depending on each and every earring. Each will be written at their product page.
BYDJO pieces are made of probably the best materials in the local market. Just like every costume jewelry, they will tarnish but with proper care, it will take a long time. I personally have several pieces that are more than 2 years old and they're still fine. It depends on how you take care of them.
With that being said, BYDJO pieces are either gold plated (meaning there is a thin layer ~usually 9k gold~ of gold plating over a base metal (brass, copper etc), gold vermeil (pronounced : ver-may) is a layer of thicker ~usually 14k gold and up plating over sterling silver or gold filled (real gold permanently bonded to a base metal (brass or sterling silver) and never solid gold.
Solid gold is very soft and to get full 9k/14k/24k gold pieces, you need to get it at your local gold jewelry store.