1. 'Small' and 'little' are both adjectives. We use 'small' to talk about the size of something. Your cat is very small. Can I have two small pizzas please?
2. We can use 'little' to refer to size, but we usually use it with another adjective to express an emotion. You're a silly little boy. Nobody's looking after that poor little dog.
3. In comparative and superlative form, 'small' is more common in British English, and 'little' is more common in American English. That's the smallest phone I've ever seen. -- British English That's the littlest phone I've ever seen. -- American English