Currently on CRM Online 2015, reached storage limit. We don't want to add more space. We keep getting this question from our customer very frequently. Our suggestion to them are following hope it helps other CRM users also to save some Dollars.
1. Email attachment should be removed or deleted. You can go to advance find and delete the attachment. Following Screenshot shows how to do this.
2. Notes Attachment Should be removed or deleted
There are lot of methods explained this that can help you to reduce space.
3. https://technet.microsoft.com/en-us/library/dn683569.aspx
You can also integrate CRM to SharePoint and move all attachment to SharePoint you can use this tool to do this.
Hope you enjoyed reading this. Feel free to comment on this