The size of user mailboxes in all Exchange Online and Office 365 service plans is doubling. Beginning today, the current 25 GB of mailbox storage is increasing to 50 GB. With this increase even the most active email users don't have to worry about mailbox size limitations-we've got you covered! There's no price increase associated with this change. Our doubling of your mailbox storage is simply part of our promise to continuously deliver value to our Office 365 customers.
Here's a summary of the new mailbox sizes by plan:
More info here