I am making one of those new 1.14 wool farms and I want it to be pretty big. Since the SMP server I play on has a max of 40 entities per chunk it has to be pretty big in terms of x and z coordinates. So basically these farms are mostly limited by the max distance in which grass spreads, I need to know how far from you grass stops spreading to find the ideal afk spot.
Grass spread is controlled by "random ticks". These happen when the center of the chunk section (16×16×16) is within 128 blocks of a player. That means that grass spreads in a pretty blocky sphere around you, with the closest point being 114 blocks away from you and the farthest 142 blocks away from you.
But I wouldn't recommend building a sheep farm this big anyway, because the tens of thousands of sheep would definitely lag the server and your client.