It is not uncommon for Dynamics CRM online customers to find themselves above the CRM file space limits and forced to pay for extra file space. Here are some options to clear up space.

1. Email Attachments
   If you are tracking emails, then it is really easy to start consuming space. I don’t recommend CRM online instance to track all emails for this reason. Attachments in particular are the issue, not the email message itself. I recommend a recurring bulk delete to clear out emails older than some time period – like 2 years. Another option is to use a third-party solution which moves the email attachments out of CRM and into a SharePoint instance (SharePoint is much cheaper per GB than CRM).
2. File attachments on notes
   This is the same as email attachments but just in another location. You can run advanced find queries against the Notes (annotation) entity and filter by file size to find the largest files. You can bulk delete them as well but it is usually harder to decide on a good time period on when to delete.  Like emails, there are third party solutions which moves the email attachments out of CRM and into a SharePoint instance.
3. Analytics app data
   Some 3rd party solutions can track a lot of information, such as analytics and email marketing solutions. Analytics apps can collect lot of information over time. Email marketing solutions may track email receipts/views which can amount to a lot of data if the email list is large. These can wrack up over time. These need to be reviewed based on the usage, often a bulk delete job will be sufficient.
4. Workflow logs
   There is a setting on CRM workflows/processes to automatically delete logs that are successful. By default, this is checked, but sometimes it gets unchecked during testing. There is a stock bulk delete job that clears up the failed logs after so much time, but there is not one set up for the success or suspended workflow logs. Suspended workflows may be valid if you have workflows that wait for something to happen, but sometimes they get stuck just wait forever. The easiest way to fix this is to set up a recurring bulk delete job for successful status logs and check on the running jobs to see if you are accumulating suspended workflows.
5. Audit logs
   This does not apply to everyone. For those who have turned on auditing, CRM will have a separate file space reserved for audit logs. If this space is running over then consider deleting past years audit logs.

MS articles:

• Free up storage space
• Bulk Delete
• Audit Logs