Rats in a world after man, likely an asteroid is gonna hit, will start to occupy the niches left behing by the extinct large modern day mammals. Some will be the size of dogs, being high in the food chain they will have few predators except for falcons. Like in the K-T extinction event they will diversify and get larger but unlike the one that killed of the non avian dinosaurs the birds will rule becoming the age of birds. Their will be plenty of food for the rats to eat like large insects. Cities might become important habitats, trees will start to reclaim the land, meadows and open woodland will be common habitats.Some will be 1.5 meters and others would be 2.68 meters ( 7.3 ft,6 inches).