Chin,
I have never found the red eyed crabs in my tank much bigger than the one in the video. I can't imagine finding one as large as the one you describe
I got it out just in time for my ocellaris pair to their lay eggs yesterday
While filming that tank yesterday, I caught a glimpse of the claw of another crab in some mushrooms growing near the GBTA. Don't know if it lives in the mushroom rock, but may try to take the rock out later today. From the claw, it doesn't look like the same species of crab ....but you never know