What would be household size if I want to bring my and my wife's parents to US living in same home. We dont have kids.

I and my wife, both as U.S citizens want to apply for greencard for my both parents and my wife's both parents, totally 4 persons to immigrate to U.S at the same time. I filled out 2 separate petitions for my parents and my wife did the same for her parents. Both me and my wife have our own salaries. We want them all 4 to live with us in our house. 1) what will be the household size I have to put in I-864? 2) my salary is above 39.962 dollars annually, the 125% poverty line for a household of 6 (if it will be the household size including me, my wife, my parents, her parents). Of course, I did not include the monthly mortgage for house and payments for cars, and ... (My salary which is > 40,000 dollars is pre-tax, pre-mortgage, pre-payments). Should I fill out I-864A or have my wife as a joint sponsor?