This is a pretty big deal for SharePoint Online. Up until recently you could only have (for Enterprise plans) site collections up to 100GB in size. 100GB isn’t anything to sneeze at, but there are cases – quite a lot of them, actually – where storage accumulated over the last, say 10 years, exceeded that quite a lot. There are also folks who use SharePoint Online for storing lots of high-res imagery, and in any scaled operation, 100GB limit starts to become restrictive.
Good solution architecture and planning can overcome such constraints, of course. In many cases it is preferable to segment a solution across site collections – regardless of storage limits. There are scenarios, however (long term archiving), for example, where it may be preferable to let storage grow (and grow).
Microsoft announced (almost a month ago, actually) that SharePoint Online now supports site collections up to 1TB in size. In addition, you can grow your tenant almost without limits – creating as many of those 1TB site collections as your budget and business will allow (up to 10,000, which is the current limit).
Here’s the original blog post explaining the details: http://blogs.office.com/2014/03/14/sharepoint-online-announces-1tb-site-collections-and-unlimited-tenant-storage-scale/