Litter box Bit Tip: The Right Size

My cat has a marking problem, we got him from a shelter 6 years ago, he is 7 1/2 now, he has been fixed but he wasn't fixed until we adopted him and he was a year and a half by then. We have tried everything. We feel we need to have him put down. we have taken him to the vet and it is nothing medical. He does use his litterbox. He has ruined 4 carpets now. we have tried the pheramones, the boundary stuff, everything. he doesn't pee all over the house just on our new carpets we get. It's so frustrating. i use natures miracle, it's ok but he will still pee. We are at our last rope with him. other than that he is an awesome cat.