The answer to this question varies according to the zoning of the property.
In the Agricultural Limited District you must have five acres.
Reference: Article III table 3.03 of the Unified Zoning Ordinance
In the Agricultural Intensive District you must have 40 acres for livestock.
You may apply for a variance on this acreage requirement.
Reference: Article III-1, table 3.1-03 of the Unified Zoning Ordinance.
Livestock are not permitted in "R" "C" or "M" zones.