New york state does not have an acreage limit on farms to get the exemption, but does want to see $10,000.00 a year in sales to qualify for the exemption ($2,000 of which can be forestry related). They do have a provision for transititioning hobby farms to commercial operations over a 3-5 year period. Apparently you just have to submit a completed form to the vendor to get the exemption, but the vendor is not required to give it to you, in which case you would need to apply for a refund.
It's pretty clearly laid out
here.
Apparently what is considered Agriculture is fairly broad:
Farms that undertake commercial activity in one of the following areas are eligible for the exemption:
Agriculture; including livestock, dairy, poultry
Floriculture
Horticulture; includes nursery stock, ornamental shrubs, ornamental trees & flowers
Viniculture
Viticulture
Aquaculture; including fish, fish products, water plants & shellfish
Silviculture; including the growing & harvesting of trees, bushes & shrubs
Commercial horse boarding operations are also included. They are defined as enterprises of at least 7 acres and 10 boarding horses that receive $10,000 or more in gross receipts annually from the boarding of horses. This does not include facilities that are primarily used for horse racing.