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.

Spotify: cache setting
The size otion however is hidden in the config file, and can only be changed using atext editor. Before changing this file, end the Spotify client.
vim .config/spotify/prefsAnd 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:
storage.size=256Save the file, and start Spotify. It will clean up the cache.