Certainly (state) size matters, but it doesn't seem to be everything. Why are counties where they are? How much have they changed since states were incorporated?
There are currently ~3100 counties and parishes in the US, any sort of comprehensive answer on how they got their boundaries would not just be book length but series length.
For your chances of getting better answers is there perhaps a particular state or group of counties that caused you to ask the question?
Not quite the same, but I saw this question and it does relate to this answer I wrote about Southern Country Courthouses as I touch on size there, so it might be of interest.