I have had a small one (about 1-1/2" - 2") for over a year and he is fat and happy. I never see him eat anything I put in. He is constantly picking off the rocks. I have a 90 gal. tank w/20 gal. refuge and 20 gal. sump. I have around 120 lbs. of LR.
I had a large one (3"+) that would eat out of a turkey baster. He only lasted two months.
I'm not completely sure of the exact size requirements. My LFS always says they need a minimum of 75 lbs. of LR. I built my setup with the intent on keeping one. I had the tank running for 9 months prior to getting him. I had loads of copods and anthipods living in my refuge and purchased some live mysis, which now breed in my refuge.
Plus, you must not have anything else that is a pod eater. Like a 6-line. I you do want one, make sure your tank is mature enough and you have a good refuge. JME