Hello Paul and welcome to the forums. We need to know what A360 product you are using to better answer this question. Can you share with us a screenshot including the URL of the WEB Site after you have signed into the site? If you look at the Overview page, you can see that for BIM 360 Team and Fusion Team, you get 500 GB per Team Member. The space is pooled, so if you add more members, you get more space.
http://www.autodesk.com/products/a360/overview
If you have A360 FREE, then it's 5 gigs of space and one project, there is no adding more space.
If you have A360 Drive, you get 5 gigs for FREE and 25 gigs if you are on a supported subscription.
Hope this helps and please let us know more details about your A360 account.
Thanks again for posting to the forums and have a great day.