The Space Usage screen consists of two parts - the Database and Tables part.
Database Space Usage window
The Database part clearly shows the files used, and the current fill <-> unused ratio. The same goes for the transaction log space - in the above example screenshot, you can see the total database size is 722 Mb, the transaction log taking the larger part of that space, namely 597 Mb. The Data part takes 122 Mb, with about 12 Mb for index data.
The gradient bars show the amount of used space.
Space Usage for Tables
The Tables part shows an overview of all tables in the database, including row count, reserved space, occupying data space etc etc. The columns can be sorted by clicking the column headers.
The Average Row Size is calculated as data (KB) / rows... eg - table Aanvoerder:
(24 * 1024) / 465 = 52.8 bytes/row.