Just a note of attention: 

Many people find that the system is taking a lot of space because they have never purge any message. In that case, do not purge all the messages at one! Purging messages will create journal entries, so if you do a very big purge you can expect a lot of journal growth and you have to take it into account. 

My recommendation would be to purge in chunks, controlling how much journal files increase and how much space left you have.