In Office 365, the provided mailbox storage per user is pretty large. It may be useful to know how storage your users are using. It will tell you if you are in the clear, or if you need to take action.
Open portal.office.com and go to Admin -> Reports -> Usage
Near the top, select the drop down for Exchange -> Mailbox Usage and choose Mailbox (number of provisioned mailboxes), Storage (total amount of storage in use), or Quota (useful to see individual users over quota).
If you are not able to navigate to these reports, you may not have the appropriate access. The above screenshot has been made using global admin access.