See the links below on how to shrink the size of the WSS_Logging database.
Step 1:
https://guidesharepoint.wordpress.com/2014/03/03/how-to-reduce-the-size-of-logging-database-and-how-to-purge-the-old-data-from-logging-database/
If you need to shrink the database after you have changed the Sharepoint retention settings:
Step 2:
Read the following article on why Shrinking Databases is potentially bad.
http://www.brentozar.com/archive/2009/08/stop-shrinking-your-database-files-seriously-now/
Step 3:
The Books Online article for Shrinking Databases and Database Files
https://technet.microsoft.com/en-us/library/ms189035(v=sql.105).aspx
Remember to look at using the Simple recovery mode to avoid growing the transaction log. If you do change the recovery mode remember to backup the databases afterwards to avoid broken transaction log chains.
Considerations for Switching from the Simple Recovery Model
Step 4;
The Shrink Database action can be a slow process. Use the following TSql to check on progress:
SELECT percent_complete, start_time, status, command, estimated_completion_time, cpu_time, total_elapsed_time FROM sys.dm_exec_requests WHERE command = 'DbccFilesCompact'