That is a matter of opinion. Little sis gets less willing to play all the time. She gets more girly, and has an increasingly short attention span. I think her main complaint is the thought of spending hours doing one thing.
I have a good friend who also plays, but only if he's over at my house long enough.
As to size, I'm up to nearly 70 minifigs per army for the next battle.
I'm thinking either an urban city setting, or a D-Day style amphibious landing. The hard part will be getting another player and enough time.