We have the wire cage for Cody, but of course it is a smaller size but we got it at PetSmart. They have many different sizes and the the box shows what size dogs it will fit. We had the smallest size for Cody at first and they said it should fit him all his life but when he got a little bigger I wasn't happy with that size so we got the next size bigger. Maybe when your boxer is laying down in his favorite position you could measure him roughly, the length from head to end of tail and width from head to end of his feet and that might help determine the size, also measure him when he is standing or sitting to get the height that is needed. I like the dogs to be able to stretch out in their cage and not bump their heads when they sit or stand. Out neighbor has a nice one for her dog who is pretty good size, it opens on the side and on one end and it has a devider. I like the wire cages for the dogs because they can see out of it and see what's going on. Good luck!