I don't believe that in the default site there's any page that tells you.
If you're running Windows, you can download Photosynth's original desktop application and sign in with your Microsoft Account that you use here on the site and it should tell you how much space you've used.
Alternately, if you have Google Chrome installed, you could add Henri Astre's Chrome extension: http://bit.ly/photosynthchrometoys which will change the layout of the old Photosynth 1 site a bit and add some pages as well as adding three new tabs to your profile page.
His Chrome extension uses the Photosynth Webservice API to fetch your currently used amount of space and displays it on the 'My Info' tab.
(Again, you'll need to be using Chrome with Henri's extension installed to use that link.)