I would call USDA - I wonder if they will cover it if the barn is not the only structure on the property?  I know, also, that the acreage and property need to be within the norm for the area.  So if everyone in the area where you want to buy have only .25 acre properties, 5 acres might not work for USDA, but if everyone has 4-5 acres it would work.  Do you have a contact at your USDA you can ask these questions?  Seems like they could advise you...because from what I understand, USDA differs depending on the state and even county where you're buying...