"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?
Your cat is very small.
Can I have two small pizzas please?
We can use "little" to refer to size in the same way as "small", but we usually use it with another adjective to express an emotion.
He's a clever little boy.
Nobody's looking after that poor little dog.
He's a clever little boy.
Nobody's looking after that poor little dog.
Additional points
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
