Farmerbob's formula is correct (you beat me too it!).
This produces the following results:
10 player tribe = 7,850 square meter area (100 meter diameter)
20 player tribe = 17,662 square meter area (150 meter diameter)
30 player tribe = 31,400 square meter area (200 meter diameter)
40 player tribe = 49,062 square meter area (250 meter diameter)
50 player tribe = 70,650 square meter area (300 meter diameter)
This scheme for tribal areas encourages tribes to have only one identity instead of breaking up into sub-tribes with each having their own area because a 20 player tribe will have more area then two 10 player tribes. This pattern continues no matter the number of players in a tribe.