Early on, efficiency is important. You can run 10x15 or 11x15 beans(maybe even corn) with one farmer, especially if you manually harvest at 80-85%. Later, as your city grows you will eventually be using other less efficient crops and moving them around due to infestations. You will also eventually have masses of unassigned laborers. At some point it just becomes more effective to run all 15x15 fields with 2 laborers.

with 1 farmer on a 10x10 field I received 60% of the crop. currently I am working on 15x15 (largest9 farms with 2 farmers, while there are abrns and houses around it, and if I have early snowfall, and the early spring is too cold, I only get 87 % of the crop

Ive found that using a 10x15 farm with one farmer works well.