Hi
1 pasture worker per 20x20 or any less but pasture size 20x10 will give same amount of milk/meat/wool like 20x20??
A larger pasture will hold more animals, more animals means more milk/meat/wool/whatever.
What size to use depends on what kind of livestock you have.
Here's a useful reference tool: https://banishedinfo.com/tools/size-calculator
Whatever size you decide on, you should make it large enough to hold 10 of your livestock, that is the fewest that will allow you to split them into a second pasture later. With sheep for example, 16x10 is good for that.