I don't think the scale of the farm has much to do with anything. If anything the big boys probably have more modern equipment that will do a better job delivering the chemical where they want it.
Let me say that I am mostly a cattle guy and just do a little farming, but when I spray weeds with roundup in my fields and I miss a 6 inch wide strip, there will be 6 inches of healthy weeds there after the rest of the shit dies. To me that is a pretty good sign that the chemical is going pretty much where it's supposed to, and my sprayer is far from high end. I'm sure it varies widely depending on what crops are being grown and all, but in my non-scientific opinion if there's not enough chemical drifting around to kill weeds a couple inches from the end of the sprayer boom, it's hard for me to believe that there will be much getting into your house.
You can find a single study that shows just about anything will kill you/make your kids retards.
EDIT: I just noticed that you said they are growing soybeans. If so they are probably roundup-ready so it's pretty likely roundup will be what they are spraying them with which makes my example pretty relevant.
