My wife and I own a 670 acre farm, live on it, and make our livelihood from it. All of the acreage is contiguous surrounding the dwelling unit. If we sell it, our capital gain will be less than the $500K exclusion allowance for a principal residence, but I'm wondering if there is some limit on how much acreage can be bundled with the house for purposes of determining the capital gains tax exemption. Will some of the land have to be treated as real estate held for business purposes, and thus not eligible for the exemption?

No, the farm is considered to be your home, so the entire farm is covered by the $500K exemption.