Again I would stress the use of a quarantine tank. Otherwise you're risking your new one as well as all the others! Also you better fatten the new one quickly, or he'll be somebody's dinner real soon!
He's 4- 4.5 inches. The bigger ones are about 6 inches. He is a great eater. Been gorging himself on raw shrimp and frozen foods. He should fatten up in no time. He was brought up on just blackworms. I keep them all full and fat and they are all getting along great!