You could probably get a bit more by starting the first harvest at about 80% ripe. I let the first harvest run without intervention and started the second when the frost warning showed up. I also made sure the workers were educated and in place before the thaw.
If you turn the buildings sidewways so, say the house and barn can serve multiple fields then you get what appears to be about a 1% drop for each square that has to be traveled even with stone roads. Now admittedly I was testing with absolute best conditions in both case : housing, well and barn with pies (it makes a difference), charcoal, warm clothes, and otherwise empty (and kept that way by donkeys) all DIRECTLY outside the gate to the field, door to door, no gap.
If the latter then, at least in previous balances for fields I got best numbers with 13x14 field. First do you mean ideal in terms of food per square of land used and resources for construcion or food per worker per year?