What are the dimensions of this piece and does it come with a part that goes into the socket?

The dimensions of item # MVMK are 8" x 4" x 4". There is a rubber ball that fits into the swivel socket, which is sold separately. The item # for the ball is item # 8555-B510-08, and is a special-order item. Please contact customer service if you need to purchase this item. Please note the rubber ball if purchased alone will need the matching back plate, item # 8555-0526-0A. This is also a special-order item and will require you to reach out to customer service for a quote.