First of all, you
should probably be looking at how long you want your pastures to rest in between grazing days?
Here is how you could calculate it in an good scenario. I don't make any guarantees on the soundness of my math, and how well things work depend on a lot of variables due to your climate, season length, and pasture health. It's simply an example.
Lets start with 6.25 acres, since that results in rounder numbers.
If you took a 25 day rotational cycle between grazing days,
You'd split that pasture into 25 sections of 1/4 acres each. You can make a permanent
fence for each of these paddocks. Or you can mark the corners of each .25 acre section with a flag or something, and use electric fencing for your animals, moving them to the next markers each day.
Assuming your pasture generally has 2,000lbs of forage per acre(this depends on how good your soil is and the growth habit of the grasses and plants there) and
60% average feed use(40% being patches being trampled on by animals, pooped on, or simply avoided)
you should get 300lbs of forage eaten by the animals per paddock.
Taking the general rule that an animal eats 3% of it's body weight each day, you should be able to stock animals that weigh up to 10,000 lbs combined, or rather 10 Animal Units on the pasture.
Animal units are 1,000 lbs each, so 1 cow or 1 horse, 10 sheep, 150 chickens, 65 geese etc... are all one unit each. Generally, just look up what the average adult weight of the animal is and divide by 1000 to find out the Animal Unit.
These links should help.
http://msucares.com/crops/forages/newsletters/08/6.pdf
http://www.thecattlesite.com/articles/2019/pasture-management-measure-forage-dry-matter/
http://www.mda.state.mn.us/animals/feedlots/feedlot-dmt/feedlot-dmt-animal-units.aspx
http://sugarmtnfarm.com/2007/10/12/how-much-land-per-pig/ This site caters only towards pigs, but has a large amount of information in general about rotational grazing.
The main problem with muti-species grazing is
water. You need something large enough for a cow to drink from, small enough to stop a duck from making a swimming hole out of, shallow enough to stop a chicken from drowning, and sturdy enough for all the abuse pigs give them. Standard stock tanks just don't work.