So I have a fusion drive for 1 Tb (so it's actually around 1,1 TB). Then I tried installing Windows using Bootcamp and i failed. Now i'm stuck with a major issue: my disk shrunk to 800 gigs and nothing is helping me. The Bootcamp assistant is showing an error:"The startup disk must be formatted as a single Mac OS Extended (Journaled) volume or already partitioned by Boot Camp Assistant for installing Windows". The disk util is of no use at all (Buttons of resizing aren't working, I mean the "-"). Can someone pls help me.

iMac (27-inch, Late 2013), OS X El Capitan (10.11.2)