As the post you were replying to said, we did change the formation. Fellows in particular was playing far higher up the pitch, with Stephens covering the right side of defence.
Also, it's not the height of our centre backs that's the issue. It's the rest of our team, which we are trying to compensate for by playing three centre backs. Even then, Millwall had a significant height advantage over us. Outside of those 3, I'm not sure we even have anyone else that's much taller than about five foot ten