I try to go by 10% of the body weight at least while they are young. Once they are adults, I just give them a standard sized small rat. Some of the girls get two. The boys get one. If their girth can handle a medium, then you could bump them up. For a 600g male, I'd just stick with smalls for a while. You don't need him packing on size. I have a 1000g male who gets a single 40-50g rat 1x per week. He doesn't complain.