A: The answer depends a lot on what you mean by “a 4 month old kitten”. If you mean a kitten that is 4 months old, then it turns out to be about the length of an average human hand from wrist to fingertip. That’s not much at all! If you’re talking about a full-grown cat, then they tend to reach their adult size after approximately 1 year and grow steadily thereafter. In other words, when people talk about how big kittens are at 2 or 3 years old, this is actually pretty close to being correct.