We have scorpions in Arizona. We have big scorpions and we have small scorpions. The rule of thumb is avoid the small ones: the smaller they are the more dangerous (venom-wise). Usually, the big ones are harmless -- scary, but harmless.
However, according to this article the planet's oceans used to be populated by a scorpion-like arthropod that seems pretty scary: It was more than eight-feet long and had claws that could rip off your head (see image).