The Spotify client for Linux, by default, will eat away a GB of your precious disk space. Since I mostly listen to random playlists, and rarely ever hear a song twice, there is no need to store all the songs on my hard disk.
Unfortunately the only option you can change in the settings menu (go to Advanced Settings) is the place of the storage folder, but you can't change the size there.
The size otion however is hidden in the config file, and can only be changed using a text editor. Before changing this file, end the Spotify client.
And change the line with "storage.size" into the new cache size you want. The size is measured in MB, therefore 1024 MB (the default setting) is 1 GB. Personally, as described at the beginning, I don't need much cache size, and changed it to 256 MB:
Save the file, and start Spotify. It will clean up the cache.