I picked this info up somewhere alomg the way. The requiredArea is per animal, so 10 cows would need a pasture of at least 10x20 (matching the 200 figure
@theonlywanderer mentions).
vanilla cattle : int _requiredArea = 20;
vanilla sheep : int _requiredArea = 16;
vanilla chickens : int _requiredArea = 6;
RK Bisons : int _requiredArea = 23;
RK Longhorns : int _requiredArea = 21;
RK Fresians : int _requiredArea = 20;
RK Pigs : int _requiredArea = 16;
RK Goats : int _requiredArea = 14;
RK Ducks : int _requiredArea = 7;
Regarding CC pigs and llamas, they also seem to be at 16. Leghorns also come in at 6.
Hope this helps.