The issue of allowing motion inactivity to turn off lights is always a challenge. You gotten a lot of good suggestions, I’d just like to throw in a couple of additional points.
One) for a room of that size, definitely consider using multiple motion sensors all assigned to one zone. That way activity on any sensor will register as activity in the room and you don’t have to be quite as careful about placement. 
@Mike_Maxwell has an excellent smartapp for exactly this purpose, very popular:
Smart Zone motion detector (Zone Motion Manager) [Deprecated -- no longer supported]
Two) even so, there are some households where people just don’t seem to move quite as much and may not be detected. Some people will address this by adding a second kind of device into the mix. You could use a pressure mat under a sofa cushion (although that doesn’t catch kids playing on the floor), you can use an open/close sensor on a cabinet or door if that fits your particular usage pattern. That gives you a lot more options.
Mike also has a very clever universal device type handler which would allow you to have any other sensor report as a motion sensor and be combined in your zone from option one above.
So those are just some additional options.