Formal:
* J'ai six personnes dans ma famille. (This is the most literal and grammatically correct way to say it.)
Informal:
* On est six dans ma famille. (This is a more natural and conversational way to say it.)
* J'ai une famille de six. (This is another informal way to say it, focusing on the number of people in the family.)
You can also use a variation depending on the context, like:
* Il y a six personnes dans ma famille. (There are six people in my family.)
* Nous sommes six dans ma famille. (We are six in my family.)
Remember to adjust the pronoun to fit the situation (e.g., "on" for "we", "nous" for "we").