Family of 6 here too.
We have a ninja 451 so the 9L dual with meat probe.
I can fit a 2kg chicken in one side. The best feature is the sync setting so you can set it to cook the other side at a diff temp and time and both sides will finish at the same time.
I also have a cheaper tower single basket 4L one that I had before I upgraded to the ninja. I still occasionally use the dual and the single, especially if we're having a particularly bad, freezer tea 😂
I only use the oven now if I'm making yorkies or cooking pizzas.