County tax records are notorious for being incorrect as to the size of a building. Measure it yourself. If the measured size is egregiously smaller than what the county has, you have grounds to appeal. You might also find a math error. The most important test of the fairness of your assessment is not the market value, but the relation to the assessments on other properties in the area of the subject property.
Many states publish a valuation manual that assessors are supposed to use and follow.
Possibly all the properties in an area are over-assessed, not just yours. If you are a Realtor, you know full well how to do a Comparative Market Analysis. If you aren't a Realtor, call one, he or she can do the job for you quickly and at no cost. Perhaps some part of the property should be excluded from the property tax because of its type. Ownership or use could make it exempt.