The measurements can be interpreted this way:

W 8" / D 26" / H 18" :

Each piece of glass is 8" in diameter. The fixture is 26" wide from the outermost edge of the left glass shade to the outermost edge of the right glass shade. It is 18" tall from the bottom of the glass to the top loop where the chain is connected. The fixture comes with 10 feet of prewired chain that can be cut down to your desired length, so the overall height depends on the length of chain you use.