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

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

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.