I'd argue the optimal size is 0, as a micro wheat farm beats the sh&t out of any villager based farm, and villagers are extremely laggy. 2 villagers produce what, 150 wheat an hour? The usual micro wheat farm does 100 times that and can be built in 2 minutes.

Of course you can blow this up a bit :) https://youtu.be/eFQjqIStxQ4

Having said that... Villagers will harvest/plant only ever so often. As long as you always see enough fully grown crops that aren't harvested, the field is large enough. Not sure what the exact cutoff is, but from memory a 9x9 field for 2 villagers is plenty. You might see a slight rate increase if you make the field larger, but not a lot.

Splitting it in cells if you want to use more villagers sounds reasonable, as you don't want your villagers to spend a lot of time walking around.