If you need to know how much disk space is used by each database on an SQL Server instance you can view a graphical representation via the Server space visualizer.

From the Workspace select the server you wish to view details about, right click and select Server Database Space from the Visualize menu.

Alternatively you can select Server Database Space from the Visualize section of the Start page. This will display a dialog allowing you to select the desired server.

Additional information is presented in a tool tip when then mouse is hovered over a database segment.

You can also drill down into a database by clicking on a database segment (see visualizing Database space usage for more details).