1. Login to cPanel account of your hosting plan or mail plan as mentioned in https://secure.lightningbase.com/index.php/knowledgebase/6/How-to-Access-cPanel.html
2. Click on Disk Usage under “Files” at top or Under “Statistics” in the right hand side of the panel.
3. It will show the list of folders/files and their space usage.
4. Scroll down and you will see an option Sort directories by Name and Size, choose size and it will sort the folders as per the size and it will help you to track down each folders and subfolders and their space consumption.
5. You can then remove unwnated files, old unwanted backup files etc to free up the space.