According to LiveSide.net, Microsoft Windows Live Hotmail has offered 500GB for storage space. But it is unlikely, anyone needs a 500GB inbox especially with the limitation of mail attachments. Microsoft still can brag that they offered the most space.
On the bottom right of the picture, you can see it reads Max: 512000MB (500GB)
From Office Outlook Connector, it show the size of Hotmail mailbox as well.