Which of the following sentences are correct to describe a large family?
1. They're very crowded.
2. They have a crowded family.
2. They're very populous.
3. They have a populous family.

Question

Answer

You can just say: " extended family". crowded and populous are adjectives that would describe areas/ places...

Answer

They are all inappropriate - see Dima's reply and my comment on Dima's reply.  Do you want synonyms for "large"?

Answer

An extended family just refers to members outside of your immediate family such as your uncles, aunts, cousins, grandparents, etc.  A large family is just described as a large family or a big family.  You could say they have a crowded house which may mean they have a small house or a lot of people living there.  Populous you would use in reference to a city or country which can mean a large and or dense population.