Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
Double click the Time Capsule in Finder and look at the bottom of the Finder window. You will see the remaining space indicated there.
 
20111110-j8cd6bqe25b9p2fpbm1qkd889p.jpg



20111110-jbfit7cjafpfrfdn26feg263s1.jpg


Start Finder and select Time Capsule on the left then double click the Time Capsule folder I marked in my first screenshot (you may have named your something else). That will open the Time Capsule partition and you will see the remaining space at the bottom (second screen cap).
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.