WASHINGTON, D.C. (AP) – They just needed some leg room: New research shows the great dinosaur die-off made way for mammals to explode in size — some more massive than several elephants put together. The largest land mammal ever: A rhinoceros-like creature, minus the horn, that stood 18 feet tall, weighed roughly 17 tons and grazed in forests in what is now Eurasia. It makes the better known woolly mammoth seem a bit puny. Tracking such prehistoric giants is more than a curiosity: It sheds new light on the evolution of mammals as they diversified to fill habitats left vacant by the dinosaurs. Within 25 million years of the dinosaurs’ extinction — fast, in geologic terms — overall land mammals had reached a maximum size and then leveled off, an international team of scientists reports Friday in the journal Science. And while different species on different continents reached their peaks at different points in time, that pattern of evolution was remarkably similar wor