I'm no biologist, but I think scorpions may be subject to the same size limits as insects, due to size:mass ratio problems. (Basically, if they got too big, I believe their exoskeletons would simply collapse under their own weight.)
http://en.wikipedia.org/wiki/JaekelopterusJaekelopterus rhenaniae ("Otto Jaekel's wing from the Rhineland") is an extinct species of the Eurypterida (sea scorpions). At an estimated length of 2.5 meters (8.2 feet), it is one of the two largest arthropods ever discovered (the other is a giant millipede-like animal, Arthropleura, although which animal was larger is unclear)
Also, see image for the largest arthropod currently existing..... *shudders*
