Mine is about 5.5"W x 10"H x 9"D.

For spacially challenged: It about the size of a piece of notebook paper, looking at it from the side.

Mine is wired into the network.

A bonus is that it automatically backs up all computers connected to it nightly (if they are on) and stores the last 3 days / 3 weeks / 3 months of backups.
Another bonus is the "raid" setting can be adjusted per folder. For example, my digital photos are duplicated across drives so that if any one drive fails I don't lose everything. It takes up a lot of space so duplication of music and video are prohibitive.