It needs to eat a lot, stay in a warm area, and have good insulating fur or feathers. Amphibians and fish, reptiles and insects, all deal with being small by metabolizing at the temperature of the environment. A small cold-blooded animal can warm up quickly on a sunlit rock, and requires little food to stay alive. This is certainly efficient. They need to find refuges safe from predators as the environment cools.
Large Animals Being large also has consequences.
Large animals can hold heat well, but may have trouble getting rid of body heat in very hot weather. This is an important problem, because proteins break down if the body gets too hot, and the organism may die if overheated. Many animals have developed ways of getting rid of extra heat by sweating or panting. Elephants can dissipate heat from their large, thin ears. There is some disagreement about whether the dinosaurs were warm blooded or cold blooded, or in some intermediate stage.