1 Each user initially receives 100 GB of storage in the archive mailbox. When auto-expanding archiving is turned on, additional storage is automatically added when the 100 GB storage capacity is reached. For more information, see Overview of unlimited archiving in Office 365. See the Office 365 Roadmap for details about availability.
2 To access a shared mailbox, a user must have an Exchange Online license. Shared mailboxes don’t require a separate license. However, if you want to enable In-Place Archive or put an In-Place Hold or a Litigation Hold on a shared mailbox, then an Exchange Online Plan 2 license or an Exchange Online Plan 1 with Exchange Online Archiving license is required. If you enable In-Place Archive and auto-expanding archiving for a shared mailbox, additional storage is automatically added when the 100 GB storage capacity for the archive mailbox is reached.
3 Resource mailboxes don’t require a license.
4 In-Place Archive isn’t included in Exchange Online Kiosk. However, it can be purchased as an add-on through Exchange Online Archiving. For more information, please see the Exchange Online Archiving Service Description.
5 Site mailboxes are created and managed in SharePoint Online. For more information, see Prepare for using site mailboxes in Office 365.
6 You are limited to 1000 public folder mailboxes, and the maximum total size of all public folder mailboxes is 50 TB.
7 In-Place Archive can only be used to archive mail for a single user or entity (such as a shared mailbox) for which a license has been applied. Using an In-Place Archive as a means of storing mail from multiple users or entities is prohibited. For example, an IT administrator can’t create a shared mailbox and have users copy it (through the Cc or Bcc field, or through a transport rule) for the explicit purpose of archiving. Note that a shared mailbox that multiple people use does not actually store email for those individual users. Multiple users have access, and they send email as the shared mailbox. Therefore, the only emails stored in the shared mailbox are those sent to or from it, as the shared mailbox.
8 If you have created a retention policy in Exchange Online, messages will be automatically moved to a user’s archive mailbox only if the user’s primary mailbox is larger than 10 MB. The retention policy will not run automatically for mailboxes that are smaller than 10 MB.

From <https://technet.microsoft.com/en-us/library/exchange-online-limits.aspx#StorageLimits&gt;