I heard they were real are you sure? They actually label them jurassic crabs. I do know that horshoe crabs get big though I caught some really big ones in mexico in a lagoon by are condo letting them go right after of course I even saved two that were badly tangled together in netting.
The mini-horseshoe jurassic crab is not a horseshoe at all, or even a limulid.
Triops, AKA Opus or Tadpole Shrimp, do indeed only get about an inch long, and usually less. You probably won't be able to keep them alive in a normal aquarium anyway, so lets move on to the horsehoe crab, Limulus polyphemus, which isn't a crab at all, but a creature more closely related to spiders. These are often sold in shops as youngsters only two or three inches across, and they usually don't survive very long. When they do survive, though, they can be a real terror in the tank.
The common names of things aren't very useful sometimes.