This increase to a 50 GB mailbox per user benefits Exchange Online and Office 365 service plans, including: Exchange Online Plan 1, Office 365 Small Business, Midsize Business, Enterprise E1, Government G1, and Education A1. If you have one of our premium service plans (Exchange Online Plan 2, Office 365 Enterprise E3 and E4, Government G3 and G4, Education A3 and A4), you already enjoy unlimited email storage through your personal archive, but now your default primary mailbox size is increasing to 50 GB. Kiosk user mailboxes are doubling in size, too-from 1 GB to 2 GB.
http://technet.microsoft.com/en-us/library/exchange-online-limits.aspx