Crop field sizes range from 5x5 to 12x12.
The cost of labor is proportional to the number of tiles (5x5 => 25 tiles, …)
The expected yields is also proportional to the number of tiles.
So optimizing means having the best ratio “tiles per farmer”.
The biggest ratio is for 5x6 (that still costs a single farmer => 30 tiles for a single farmer)
Then come 5x5 and 5x10 with 25 tiles per farmer.
The worst being 5x7 with 17.5 tiles per farmer.
Have you thought that less tiles per farmer is better? Farmer spend more time for each tile and care for the culture. There sometimes farmers fail to harvest crops and they spoil right in field.
Well I started with fields of 5x5 (25 tiles per farmer) and crops failed maybe once or twice because of heat or frost, but not spoiling (the wagon does the job apparently).
A 10x10 field is the perfect size for a herd of cattle to graze on a season of clover.
But 10x10 (5 farmers) is 20 tiles per farmer.
I can suggest putting 4 crop fields 5x5 next to each other (4 farmers → 25 tiles per farmer), so you have the best of both worlds in this case. Haven’t had the time to experiment grazing on crop fields unfortunately.
Or 4 crop fields 5x6 to form a 10x12 would still be nice I guess.
I only use 5x5 tiles. This way I can plant much more variable, I get more different harvests in a year from mostly everything. I don’t understand why a lot of youtube lets play players often place very big fields where you can only plant one, max two vegetables or grains.
Also smaler fields can be organized better if fertility goes down, you have enough other fields which still produce food or grains 
Of course you can plant more of the same if needed. You are more flexible with smaller tiles, in my opinion.
Edit: there is an option to combine fields. But I don’t know if you also can devide/uncombine afterwards, what would be better. But if you place a big field you cannot devide it into smaler tiles (at least I don’t know for sure, have no big ones)
Small fields produce less food. Just harvested yeld momentally consumed by settlers and your storage empty again. Although town could survive whis 1-2 month food stock, big fields could be managed same way as small ones and grant more food need to increase population.
Eachone choise field size depend of possibility.
Yes, but you can also plant more of the same if needed on smaler tiles. You are more flexible with smaller tiles, in my opinion. If fertility goes down you are more flexible, can switch between. The good thing is, everyone can do it the way they want
At least in the beginning. I think you can combine smaler fields to bigger ones later on. At least thats how I understand the option extent , haven’t tried it yet?
I use small 5x6 tiles. More flexible with crop rotation. One thing that bother me is how compost is implemented. Does small and big fields get same fertility boost or is dependent on filed size?
I use groups of 3 small fields with shifted crop rotation, so each year I have 3 different crops. I can use same method for any size fields. What about fertility: 1 compost pile spend to 1 any siza field too.
No, you can not. You can only expand the size of the current field. You can’t reduce the size, would like if you could. Plus the way you expand the field with the new grid is pretty stupid, it’s not very friendly to allow you to increase the size how you want it.
You’re forgetting 2 tiles per field used for farm storage.
The real difference is smaller - 28 tiles per farmer (5x6) and 24 tiles per farmer (5x10, 10x10 gets 24.5). Sizes that are not x5 should be avoided like hell.
If you want to use the land better - those extra 2 tiles per field do take away some land.
Also shifting the soil mix with clay/sand takes the same amount of clay/sand for both 5x6 and 5x10 (this seems to be an oversight).
Why are you suggest, that 2 tiles reserved for storage and not just visual effect?
In this case two 5x5 fields must have less effective area than one 5x10, and therefore less yeld production. Same for four 5x5 vs. two 5x10 vs. one 10x10. But it’s not like that.
You can always check how much is planted and do the math.
E.g. relatively fast-growing plants (clover, peas, maybe something else) are planted with the ratio of 8 units per tile. That’s 224 for 5x6 field (28 tiles are planted) and 384 for 5x10 field (48 tiles are planted).
Two 5x5s are definitely less effective than 5x10, and two 5x10 are less effective than 10x10.
You shouldn’t trust overly imprecise tooltips - look at the real numbers of things being planted/collected/etc every year.
Don’t forget the compost mechanic. I have a sneaky feeling it raises fertility the same, regardless the size of field. I haven’t tested it.
I do 6x6. 2 farmers per plot. 18 tiles between them. Farmers also mind the apiaries and help build new farms so having an excess of farmers has seemed to be beneficial for getting everything done.
