On a beach on Jekyll Island (off the coast of Georgia), I found a horseshoe crab that was 3 FEET WIDE, larger than any horseshoe crab I have ever seen. It was dead, and I picked it up because I wanted the shell. Unfortunately, it hadn't been dead for long, and I could hear (and smell) the decaying organs "sloshing" around inside of it. It was nasty.
Come to think of it, I think it may have been a GIANT ENEMY CRAB !
In an aquarium store, I saw live baby horseshoe crabs beings sold for a few dollars each. They were only a few inches wide.