As part of its Wave 3 update announced back in 2008, Windows Live Hotmail was to have "ever increasing storage", and now as reported by LiveSide, the current maximum mailbox size for Hotmail is 500 GB, and this includes e-mail accounts as part of Windows Live Admin Center, Office Live, and MSN Premium.
The new maximum size is shown in the two screenshots below courtesy of PicturePan2 from LiveSino:
Screenshot from MSN Explorer where you check your inbox folder sizes. It shows the max size being 512000 MB (Note: 512000 MB translates to 500 GB).
Screenshot from Office Outlook Connector, where it shows you the size of your Hotmail mailbox. It too shows the max size being 500 GB.
Supposedly, Hotmail's storage size will be "ever-increasing as required at the end of each month. The user is supposed to receive an automatic alert when their mailbox is rapidly reaching the limit before the end of month, asking them to "slow down".