The size of the dog name tag you need depends on various factors, including the size and breed of your dog, as well as your personal preference. Generally, for cats, smaller breeds or puppies, a 20mm tag may be suitable. Medium-sized dogs often do well with a 25mm tag, while larger breeds may require a 30mm tag for better visibility. Consider the space needed to accommodate the necessary information comfortably. Ultimately, choose a size that allows for clear legibility and ensures your dog's identification is easily readable.