try to keep the mice smaller then your monitors head, although they can handle prey items the same size or even a little bigger then there head.

feed him mostly insects, offering rodents 2-3 times a week. also, for variety, some other things you can feed are crawfish, fish, earth worms, reptiles, shrimp, prawns, etc. the key is too stick to whole prey items with all the nutrienst in the bones, skin, intestines, etc. in it as opposed to, say, just ground turkey.