It depends on your play style. A good rule of thumb is two acres per person, otherwise you'll have scrims that go all day. Makeup a map (google earth snap shot), put a grid on it and put up large landmark markers in each quadrant. Make sure to scour the area for large hazards (i.e. holes, water, animal nests, etc), and mark them off.
Stick with square acres, it's easier to map and navigate.
We used to play approximately 8 - 16 people on about 8 - 10 acres, and it was too small. I've also played 25 people on 160 acres, and it was way to big.
Also heavily forested area is prime area for slips, trips, and falls; so make sure everyone has a radio in case of emergency or injury.
Anyways, it sounds like fun. Playing in large wooded areas is a blast.
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